revisions.txt: be explicit about commands writing 'ORIG_HEAD'

When mentioning 'ORIG_HEAD', be explicit about which command write that
pseudo-ref, namely 'git am', 'git merge', 'git rebase' and 'git reset'.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Acked-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/revisions.txt

@@ -49,7 +49,8 @@ characters and to avoid word splitting.
 `FETCH_HEAD` records the branch which you fetched from a remote repository
 with your last `git fetch` invocation.
 `ORIG_HEAD` is created by commands that move your `HEAD` in a drastic
-way, to record the position of the `HEAD` before their operation, so that
+way (`git am`, `git merge`, `git rebase`, `git reset`),
+to record the position of the `HEAD` before their operation, so that
 you can easily change the tip of the branch back to the state before you ran
 them.
 `MERGE_HEAD` records the commit(s) which you are merging into your branch