Document System.Buffers.Binary namespace (#3457)

Author: carlossanlop

xml/ns-System.Buffers.Binary.xml

@@ -1,6 +1,6 @@
 <Namespace Name="System.Buffers.Binary">
   <Docs>
-    <summary>To be added.</summary>
+    <summary>Provides static methods for reading and writing the binary representations of primitive types from and to spans of bytes, respectively.</summary>
     <remarks>To be added.</remarks>
   </Docs>
 </Namespace>