Documentation: --no-walk is no-op if range is specified

The existing description can be misleading and cause the reader to
think that --no-walk will do something if they specify a range in the
command line instead of a set of revs.
Signed-off-by: NCarlos Martín Nieto <cmn@elego.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/rev-list-options.txt

@@ -622,6 +622,7 @@ These options are mostly targeted for packing of git repositories.
 --no-walk::
 
 	Only show the given revs, but do not traverse their ancestors.
+	This has no effect if a range is specified.
 
 --do-walk::