Automatically port ReadOnlySpan.GetPinnableReference (#3179)

* Automatically port ReadOnlySpan.GetPinnableReference

* returns suggestion by rpetrusha

Co-Authored-By: Ron Petrusha <[email protected]>

Author: carlossanlop

xml/System/ReadOnlySpan`1.xml

@@ -379,8 +379,8 @@ This method returns `default` when `array` is `null`.
       </ReturnValue>
       <Parameters />
       <Docs>
-        <summary>To be added.</summary>
-        <returns>To be added.</returns>
+        <summary>A read-only reference to an object of type T that can be used for pinning, and is required to support the use of span within a fixed statement.</summary>
+        <returns>A reference to the element of the span at index 0, or <see langword="null" /> if the span is empty.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>