rev-list: document --reflog option

This is mostly used internally, but it does not hurt to
explain it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: peff

Documentation/rev-list-options.txt

@@ -168,6 +168,10 @@ respectively, and they must begin with `refs/` when applied to `--glob`
 or `--all`. If a trailing '/{asterisk}' is intended, it must be given
 explicitly.
 
+--reflog::
+	Pretend as if all objects mentioned by reflogs are listed on the
+	command line as `<commit>`.
+
 --ignore-missing::
 	Upon seeing an invalid object name in the input, pretend as if
 	the bad input was not given.