[1068] added a note about possible future extensions to getUpdater()

of the in-memory data source client.

src/lib/datasrc/memory_datasrc.h

@@ -267,7 +267,13 @@ public:
     virtual ZoneIteratorPtr getIterator(const isc::dns::Name& name) const;
 
     /// In-memory data source is read-only, so this derived method will
-    /// result in a NotImplemented (once merged) exception.
+    /// result in a NotImplemented exception.
+    ///
+    /// \note We plan to use a database-based data source as a backend
+    /// persistent storage for an in-memory data source.  When it's
+    /// implemented we may also want to allow the user of the in-memory client
+    /// to update via its updater (this may or may not be a good idea and
+    /// is subject to further discussions).
     virtual ZoneUpdaterPtr getUpdater(const isc::dns::Name& name,
                                       bool replace) const;