Add missing documentation for new SuppressMessage scope (#3254)

* Add missing documentation for new SuppressMessage scope

The "namespaceanddescendants" scope was recently supported by Roslyn ([issue 486](dotnet/roslyn#486)).

The user guide at [/visualstudio/code-quality/in-source-suppression-overview](https://docs.microsoft.com/en-us/visualstudio/code-quality/in-source-suppression-overview?view=vs-2019#suppressmessage-attribute) already documents this new supported scope, however, it's missing here.

* Adding mairaw's changes

Use oxford commas, indicate that this scope is only supported in newer versions of Visual Studio.

Co-Authored-By: Maira Wenzel <[email protected]>

Author: fydar

xml/System.Diagnostics.CodeAnalysis/SuppressMessageAttribute.xml

@@ -325,7 +325,7 @@ The <xref:System.Diagnostics.CodeAnalysis.SuppressMessageAttribute.MessageId%2A>
   
 ## Remarks
 
-The <xref:System.Diagnostics.CodeAnalysis.SuppressMessageAttribute.Scope%2A> property is an optional argument that specifies the metadata scope for which the attribute is relevant. Possible values for managed metadata include "member", "resource", "module", "type", "method", or "namespace". For more information about scopes, see [Suppress code analysis warnings](/visualstudio/code-quality/in-source-suppression-overview#suppressmessage-attribute).  
+The <xref:System.Diagnostics.CodeAnalysis.SuppressMessageAttribute.Scope%2A> property is an optional argument that specifies the metadata scope for which the attribute is relevant. Possible values for managed metadata include "member", "resource", "module", "type", "method", "namespace", or "namespaceanddescendants" (available since Visual Studio 2019). For more information about scopes, see [Suppress code analysis warnings](/visualstudio/code-quality/in-source-suppression-overview#suppressmessage-attribute).  
   
  ]]></format>
         </remarks>