rev-list-options.txt: update --all about HEAD

This is the document patch for f0298cf (revision walker: include a
detached HEAD in --all - 2009-01-16).

Even though that commit is about detached HEAD, as Jeff pointed out,
always adding HEAD in that case may have subtle differences with
--source or --exclude. So the document mentions nothing about the
detached-ness.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pclouds

Documentation/rev-list-options.txt

@@ -133,8 +133,8 @@ parents) and `--max-parents=-1` (negative numbers denote no upper limit).
 	for all following revision specifiers, up to the next `--not`.
 
 --all::
-	Pretend as if all the refs in `refs/` are listed on the
-	command line as '<commit>'.
+	Pretend as if all the refs in `refs/`, along with `HEAD`, are
+	listed on the command line as '<commit>'.
 
 --branches[=<pattern>]::
 	Pretend as if all the refs in `refs/heads` are listed