Document that rev-list --graph triggers parent rewriting.

This may help to understand why --graph causes more comments to
be selected.

Signed-off-by: Yann Dirson <ydirson@altern.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/rev-list-options.txt

@@ -95,6 +95,8 @@ you would get an output like this:
 	to be printed in between commits, in order for the graph history
 	to be drawn properly.
 +
+This enables parent rewriting, see 'History Simplification' below.
++
 This implies the '--topo-order' option by default, but the
 '--date-order' option may also be specified.