Automatically port 5.0 System.Runtime.CompilerServices documentation (#4285)

* Automatically port System.Runtime.CompilerServices 5.0 docs

* suggestions by gewarren

Co-authored-by: Genevieve Warren <[email protected]>

* suggestion by gewarren

Co-authored-by: Genevieve Warren <[email protected]>

Co-authored-by: Genevieve Warren <[email protected]>

Author: carlossanlop

xml/System.Runtime.CompilerServices/RuntimeHelpers.xml

@@ -86,10 +86,10 @@
         <Parameter Name="size" Type="System.Int32" Index="1" FrameworkAlternate="net-5.0" />
       </Parameters>
       <Docs>
-        <param name="type">To be added.</param>
-        <param name="size">To be added.</param>
-        <summary>To be added.</summary>
-        <returns>To be added.</returns>
+        <param name="type">The type associated with the allocated memory.</param>
+        <param name="size">The amount of memory to allocate, in bytes.</param>
+        <summary>Allocates memory that's associated with the <paramref name="type" /> and is freed if and when the <see cref="T:System.Type" /> is unloaded.</summary>
+        <returns>The allocated memory.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>

xml/System.Runtime.CompilerServices/SkipLocalsInitAttribute.xml

@@ -19,8 +19,16 @@
     </Attribute>
   </Attributes>
   <Docs>
-    <summary>To be added.</summary>
-    <remarks>To be added.</remarks>
+    <summary>Indicates to the compiler that the .locals init flag should not be set in nested method headers when emitting to metadata.</summary>
+    <remarks>
+      <format type="text/markdown"><![CDATA[
+
+## Remarks
+
+This attribute is unsafe, because it may reveal uninitialized memory to the application in certain instances (for example, reading from uninitialized stack-allocated memory). If applied to a method directly, the attribute applies to that method and all its nested functions, including lambdas and local functions. If applied to a type or module, it applies to all methods nested inside. This attribute is intentionally not permitted on assemblies. To apply the attribute to multiple type declarations, use it at the module level instead.
+
+          ]]></format>
+    </remarks>
   </Docs>
   <Members>
     <Member MemberName=".ctor">