Document gitk --argscmd flag.

This was part of my original patch, but appears to have been lost.
Signed-off-by: NYann Dirson <ydirson@altern.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/gitk.txt

@@ -49,6 +49,13 @@ frequently used options.
 	the history between two branches (i.e. the HEAD and the MERGE_HEAD)
 	that modify the conflicted files.
 
+--argscmd=<command>::
+	Command to be run each time gitk has to determine the list of
+	<revs> to show.  The command is expected to print on its standard
+	output a list of additional revs to be shown, one per line.
+	Use this instead of explicitly specifying <revs> if the set of
+	commits to show may vary between refreshes.
+
 <revs>::
 
 	Limit the revisions to show. This can be either a single revision