Document the key 'unique' in messaging protocol

docs/source/development/messaging.txt

@@ -634,6 +634,10 Message type: ``history_request``::
       # 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,
+      
     }
 
 Message type: ``history_reply``::