Document the key 'unique' in messaging protocol

docs/source/development/messaging.txt

@@ -633,6 +633,10 b' Message type: ``history_request``::'
       # If hist_access_type is 'search', get cells matching the specified glob
       # pattern (with * and ? as wildcards).
       'pattern' : str,
+
+      # If hist_access_type is 'search' and unique is true, do not
+      # include duplicated history.  Default is false.
+      'unique' : bool,
       
     }