Fixed links to the ref struct doc (#4146)

Author: pkulikov

xml/System/Memory`1.xml

@@ -52,7 +52,7 @@
   
 ## Remarks  
   
-Like <xref:System.Span%601>, `Memory<T>` represents a contiguous region of memory. Unlike <xref:System.Span%601>, however, `Memory<T>` is not a [ref struct](~/docs/csharp/language-reference/keywords/ref.md#ref-struct-types). This means that `Memory<T>` can be placed on the managed heap, whereas <xref:System.Span%601> cannot. As a result, the `Memory<T>` structure does not have the same restrictions as a <xref:System.Span%601> instance. In particular:
+Like <xref:System.Span%601>, `Memory<T>` represents a contiguous region of memory. Unlike <xref:System.Span%601>, however, `Memory<T>` is not a [ref struct](~/docs/csharp/language-reference/builtin-types/struct.md#ref-struct). This means that `Memory<T>` can be placed on the managed heap, whereas <xref:System.Span%601> cannot. As a result, the `Memory<T>` structure does not have the same restrictions as a <xref:System.Span%601> instance. In particular:
 
 - It can be used as a field in a class.
 

xml/System/ReadOnlySpan`1+Enumerator.xml

@@ -52,7 +52,7 @@ Though the <xref:System.ReadOnlySpan`1> is allocated on the stack, the underlyin
 
 Unlike some other enumerator structures in .NET, the <xref:System.ReadOnlySpan`1.Enumerator>:
 
-- Does not implement the <xref:System.Collections.IEnumerator> or <xref:System.Collections.Generic.IEnumerator`1> interface. This is because <xref:System.ReadOnlySpan`1.Enumerator> is a [ref struct](~/docs/csharp/language-reference/keywords/ref.md#ref-struct-types) and cannot be boxed.
+- Does not implement the <xref:System.Collections.IEnumerator> or <xref:System.Collections.Generic.IEnumerator`1> interface. This is because <xref:System.ReadOnlySpan`1.Enumerator> is a [ref struct](~/docs/csharp/language-reference/builtin-types/struct.md#ref-struct) and cannot be boxed.
 
 - Does not include a `Reset` method, which can set the enumerator to its initial position before the first element in the span. (The <xref:System.Collections.IEnumerator.Reset?displayProperty=nameWithType> method must be implemented as part of the interface, but most implementors either throw an exception or provide no implementation.)
 

xml/System/ReadOnlySpan`1.xml

@@ -44,7 +44,7 @@
     <typeparam name="T">The type of items in the <see cref="T:System.ReadOnlySpan`1" />.</typeparam>
     <summary>Provides a type-safe and memory-safe read-only representation of a contiguous region of arbitrary memory.</summary>
     <remarks>
-      <format type="text/markdown"><![CDATA[`ReadOnlySpan<T>` is a [ref struct](~/docs/csharp/language-reference/keywords/ref.md#ref-struct-types) that is allocated on the stack and can never escape to the managed heap. Ref struct types have a number of restrictions to ensure that they cannot be promoted to the managed heap, including that they can't be boxed, captured in lambda expressions, assigned to variables of type <xref:System.Object>, assigned to `dynamic` variables, and they cannot implement any interface type.
+      <format type="text/markdown"><![CDATA[`ReadOnlySpan<T>` is a [ref struct](~/docs/csharp/language-reference/builtin-types/struct.md#ref-struct) that is allocated on the stack and can never escape to the managed heap. Ref struct types have a number of restrictions to ensure that they cannot be promoted to the managed heap, including that they can't be boxed, captured in lambda expressions, assigned to variables of type <xref:System.Object>, assigned to `dynamic` variables, and they cannot implement any interface type.
 
 A `ReadOnlySpan<T>` instance is often used to reference the elements of an array or a portion of an array. Unlike an array, however, a `ReadOnlySpan<T>` instance can point to managed memory, native memory, or memory managed on the stack.]]></format>
     </remarks>

xml/System/Span`1.xml

@@ -46,7 +46,7 @@
     <remarks>
       <format type="text/markdown"><![CDATA[  
 
-`Span<T>` is a [ref struct](~/docs/csharp/language-reference/keywords/ref.md#ref-struct-types) that is allocated on the stack rather than on the managed heap. Ref struct types have a number of restrictions to ensure that they cannot be promoted to the managed heap, including that they can't be boxed, they can't be assigned to variables of type <xref:System.Object>, `dynamic` or to any interface type, they can't be fields in a reference type, and they can't be used across `await` and `yield` boundaries. In addition, calls to two methods, <xref:System.Span%601.Equals(System.Object)> and <xref:System.Span%601.GetHashCode%2A>, throw a <xref:System.NotSupportedException>. 
+`Span<T>` is a [ref struct](~/docs/csharp/language-reference/builtin-types/struct.md#ref-struct) that is allocated on the stack rather than on the managed heap. Ref struct types have a number of restrictions to ensure that they cannot be promoted to the managed heap, including that they can't be boxed, they can't be assigned to variables of type <xref:System.Object>, `dynamic` or to any interface type, they can't be fields in a reference type, and they can't be used across `await` and `yield` boundaries. In addition, calls to two methods, <xref:System.Span%601.Equals(System.Object)> and <xref:System.Span%601.GetHashCode%2A>, throw a <xref:System.NotSupportedException>. 
 
 > [!IMPORTANT]
 > Because it is a stack-only type, `Span<T>` is unsuitable for many scenarios that require storing references to buffers on the heap. This is true, for example, of routines that make asynchronous method calls. For such scenarios, you can use the complementary <xref:System.Memory%601?displayProperty=nameWithType> and <xref:System.ReadOnlyMemory%601?displayProperty=nameWithType> types.
@@ -394,7 +394,7 @@ This method copies all of `source` to `destination` even if `source` and `destin
           <format type="text/markdown"><![CDATA[  
 Calls to the <xref:System.Span%601.Equals%2A> method are not supported. Calls to the <xref:System.Span%601.Equals%2A> methods produce either of two results:
 
-- If `obj` is a <xref:System.Span%601>, the method call generates compiler error CS1503: "cannot convert from 'System.Span' to 'object'." This is because <xref:System.Span%601> is a [ref struct](~/docs/csharp/reference-semantics-with-value-types.md#ref-struct-type) that cannot be boxed and therefore cannot be converted to an <xref:System.Object>.
+- If `obj` is a <xref:System.Span%601>, the method call generates compiler error CS1503: "cannot convert from 'System.Span' to 'object'." This is because <xref:System.Span%601> is a [ref struct](~/docs/csharp/language-reference/builtin-types/struct.md#ref-struct) that cannot be boxed and therefore cannot be converted to an <xref:System.Object>.
 
 - If the type of `obj` is not a <xref:System.Span%601>, the method call throws a <xref:System.NotSupportedException>. 
 
@@ -543,7 +543,7 @@ Instead of calling the <xref:System.Span%601.GetEnumerator%2A> method directly,
         <remarks>
           <format type="text/markdown"><![CDATA[  
 
-The `GetPinnableReference` method returns a [ref struct](~/docs/csharp/reference-semantics-with-value-types.md#ref-struct-type). It can be used for pinning a <xref:System.Span%601> in memory. It is required to support the use of a <xref:System.Span%601> within a [fixed](~/docs/csharp/language-reference/keywords/fixed-statement.md) statement.
+The `GetPinnableReference` method returns a [ref struct](~/docs/csharp/language-reference/builtin-types/struct.md#ref-struct). It can be used for pinning a <xref:System.Span%601> in memory. It is required to support the use of a <xref:System.Span%601> within a [fixed](~/docs/csharp/language-reference/keywords/fixed-statement.md) statement.
 
        ]]></format>
         </remarks>

xml/System/Type.xml

@@ -11129,7 +11129,7 @@ GetType(Array).IsAssignableFrom(type)
           <format type="text/markdown"><![CDATA[  
 
 ## Remarks
-Byref-like structures are declared using `ref struct` keyword in C#. An instance of the byref-like structure can't be placed on the managed heap. For more information, see [Ref struct types](~/docs/csharp/language-reference/keywords/ref.md#ref-struct-types).
+Byref-like structures are declared using `ref struct` keyword in C#. An instance of the byref-like structure can't be placed on the managed heap. For more information, see [Ref struct](~/docs/csharp/language-reference/builtin-types/struct.md#ref-struct).
 
  ]]></format>
         </remarks>