Document System.Buffers.Text namespace (#3673)

* Document System.Buffers.Text namespace

* Update xml/ns-System.Buffers.Text.xml

Co-Authored-By: Layomi Akinrinade <[email protected]>

* Update xml/ns-System.Buffers.Text.xml

Co-Authored-By: Ahson Khan <[email protected]>

* Update xml/ns-System.Buffers.Text.xml

Co-Authored-By: Genevieve Warren <[email protected]>

Author: carlossanlop

xml/ns-System.Buffers.Text.xml

@@ -1,6 +1,6 @@
 <Namespace Name="System.Buffers.Text">
   <Docs>
-    <summary>To be added.</summary>
+    <summary>This namespace contains types that can be used to parse and format common data types to and from UTF-8 text representations.</summary>
     <remarks>To be added.</remarks>
   </Docs>
 </Namespace>