Document System.Resources.Extensions Namespace (#2913)

* Document System.Resources.Extensions Namespace

* Update ns-System.Resources.Extensions.xml

* Update ns-System.Resources.Extensions.xml

Author: carlossanlop

xml/ns-System.Resources.Extensions.xml

@@ -1,6 +1,6 @@
 <Namespace Name="System.Resources.Extensions">
   <Docs>
-    <summary>To be added.</summary>
+    <summary>Provides classes that can read and write resources in a format that supports non-primitive objects.</summary>
     <remarks>To be added.</remarks>
   </Docs>
 </Namespace>