Document System.IO.Pipelines namespace (#3497)

Author: carlossanlop

xml/ns-System.IO.Pipelines.xml

@@ -1,6 +1,6 @@
 <Namespace Name="System.IO.Pipelines">
   <Docs>
-    <summary>To be added.</summary>
+    <summary>The System.IO.Pipelines namespace provides types that allow performing complex, high performance IO operations.</summary>
     <remarks>To be added.</remarks>
   </Docs>
 </Namespace>