Document when MemoryStream.TryGetBuffer returns false (#3634)

* Document when MemoryStream.TryGetBuffer returns false

* fix build warnings in xrefs

Author: carlossanlop

xml/System.IO/MemoryStream.xml

@@ -2016,11 +2016,29 @@ The pending operation does not support writing.</exception>
         <Parameter Name="buffer" Type="System.ArraySegment&lt;System.Byte&gt;" RefType="out" Index="0" FrameworkAlternate="dotnet-uwp-10.0;netcore-1.0;netcore-1.1;netcore-2.0;netcore-2.1;netcore-2.2;netcore-3.0;netframework-4.6;netframework-4.6.1;netframework-4.6.2;netframework-4.7;netframework-4.7.1;netframework-4.7.2;netframework-4.8;netstandard-1.3;netstandard-1.4;netstandard-1.5;netstandard-1.6;netstandard-2.0;netstandard-2.1;xamarinandroid-7.1;xamarinios-10.8;xamarinmac-3.0" />
       </Parameters>
       <Docs>
-        <param name="buffer">The byte array segment from which this stream was created.</param>
+        <param name="buffer">When this method returns <see langword="true" />, the byte array segment from which this stream was created; when this method returns <see langword="false" />, this parameter is set to <see langword="default" />.</param>
         <summary>Returns the array of unsigned bytes from which this stream was created. The return value indicates whether the conversion succeeded.</summary>
         <returns>
-          <see langword="true" /> if the conversion was successful; otherwise, <see langword="false" />.</returns>
-        <remarks>To be added.</remarks>
+          <see langword="true" /> if the buffer is exposable; otherwise, <see langword="false" />.</returns>
+        <remarks>
+          <format type="text/markdown">
+  <![CDATA[
+
+## Remarks
+
+This method returns `true` when the the underlying buffer is marked as exposable, which happens when the current `MemoryStream` instance is created with:
+
+- <xref:System.IO.MemoryStream.%23ctor>.
+- <xref:System.IO.MemoryStream.%23ctor%28System.Int32%29).
+- <xref:System.IO.MemoryStream.%23ctor%28System.Byte%5B%5D,System.Int32,System.Int32,System.Boolean,System.Boolean%29) with the parameter `publiclyVisible` set to `true`.
+
+The underlying buffer will not be exposed if the current `MemoryStream` instance is created with:
+
+- <xref:System.IO.MemoryStream.%23ctor%28System.Byte%5B%5D,System.Boolean%29).
+- <xref:System.IO.MemoryStream.%23ctor%28System.Byte%5B%5D,System.Int32,System.Int32,System.Boolean,System.Boolean%29) with the parameter `publiclyVisible` set to `false`.
+
+          ]]></format>
+        </remarks>
       </Docs>
     </Member>
     <Member MemberName="Write">