git-reset.txt: mention 'ORIG_HEAD' in the Description

The fact that 'git reset' writes 'ORIG_HEAD' before changing HEAD is
mentioned in an example, but is missing from the 'Description' section.

Mention it in the discussion of the "'git reset' [<mode>] [<commit>]"
form of the command.

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/git-reset.txt

@@ -49,7 +49,8 @@ section of linkgit:git-add[1] to learn how to operate the `--patch` mode.
 'git reset' [<mode>] [<commit>]::
 	This form resets the current branch head to `<commit>` and
 	possibly updates the index (resetting it to the tree of `<commit>`) and
-	the working tree depending on `<mode>`. If `<mode>` is omitted,
+	the working tree depending on `<mode>`. Before the operation, `ORIG_HEAD`
+	is set to the tip of the current branch. If `<mode>` is omitted,
 	defaults to `--mixed`. The `<mode>` must be one of the following:
 +
 --