Documentation: clarify commits affected by gitk --merge

Signed-off-by: NSitaram Chamarty <sitaramc@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/gitk.txt

@@ -47,7 +47,8 @@ frequently used options.
 
 	After an attempt to merge stops with conflicts, show the commits on
 	the history between two branches (i.e. the HEAD and the MERGE_HEAD)
-	that modify the conflicted files.
+	that modify the conflicted files and do not exist on all the heads
+	being merged.
 
 --argscmd=<command>::
 	Command to be run each time gitk has to determine the list of