Add missing namespace docs for System.IO.Hashing

Author: bartonjs

xml/ns-System.IO.Hashing.xml

@@ -1,6 +1,6 @@
 <Namespace Name="System.IO.Hashing">
   <Docs>
-    <summary>To be added.</summary>
+    <summary>Contains types used in computing non-cryptographic hash values.</summary>
     <remarks>To be added.</remarks>
   </Docs>
 </Namespace>