Documentation: define 'MERGE_AUTOSTASH'

The documentation for 'git merge --abort' and 'git merge --quit' both
mention the special ref 'MERGE_AUTOSTASH', but this ref is not formally
defined anywhere. Mention it in the description of the '--autostash'
option for 'git merge'.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/merge-options.txt

@@ -154,7 +154,8 @@ endif::git-pull[]
 --autostash::
 --no-autostash::
 	Automatically create a temporary stash entry before the operation
-	begins, and apply it after the operation ends.  This means
+	begins, record it in the special ref `MERGE_AUTOSTASH`
+	and apply it after the operation ends.  This means
 	that you can run the operation on a dirty worktree.  However, use
 	with care: the final stash application after a successful
 	merge might result in non-trivial conflicts.