Clarify undefined behavior for [ReadOnly]Span<T>(void*,int) constructors (#9670)

* Update Span`1.xml

For #861

* Update ReadOnlySpan`1.xml

For #861

* Update ReadOnlySpan`1.xml

Per @stephentoub

* Update Span`1.xml

Per @stephentoub

Author: colejohnson66

xml/System/ReadOnlySpan`1.xml

@@ -217,7 +217,9 @@ A `ReadOnlySpan<T>` instance is often used to reference the elements of an array
         <param name="length">The number of <typeparamref name="T" /> elements to be included in the <see cref="T:System.ReadOnlySpan`1" />.</param>
         <summary>Creates a new <see cref="T:System.ReadOnlySpan`1" /> from a specified number of <typeparamref name="T" /> elements starting at a specified memory address.</summary>
         <remarks>
-          <format type="text/markdown"><![CDATA[This constructor should be used with care, since it creates arbitrarily typed `T`s from a block of memory.]]></format>
+          <format type="text/markdown"><![CDATA[
+          This constructor should be used with care, since it creates arbitrarily typed `T`s from a `void*`-typed block of memory, and neither `pointer` nor `length` are validated by the constructor.
+          ]]></format>
         </remarks>
         <exception cref="T:System.ArgumentException">
           <paramref name="T" /> is a reference type or contains references and therefore cannot be stored in unmanaged memory.</exception>

xml/System/Span`1.xml

@@ -208,10 +208,8 @@ If `array` is `null`, this constructor returns a `null` `Span<T>`.
         <summary>Creates a new <see cref="T:System.Span`1" /> object  from a specified number of <typeparamref name="T" /> elements starting at a specified memory address.</summary>
         <remarks>
           <format type="text/markdown"><![CDATA[
-
-        This constructor should be used with care, since it creates arbitrarily typed `T`s from a `void*`-typed block of memory, and `length` is not checked. However, if the constructor succeeds in creating a new <xref:System.Span%601> object, all subsequent uses are correct.
-
-      ]]></format>
+          This constructor should be used with care, since it creates arbitrarily typed `T`s from a `void*`-typed block of memory, and neither `pointer` nor `length` are validated by the constructor.
+          ]]></format>
         </remarks>
         <exception cref="T:System.ArgumentException">
           <paramref name="T" /> is a reference type or contains pointers and therefore cannot be stored in unmanaged memory.</exception>