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

The fact that 'git merge' writes 'ORIG_HEAD' before performing the merge
is missing from the documentation of the command.

Mention it in the 'Description' section.

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-merge.txt

@@ -37,7 +37,8 @@ Then "`git merge topic`" will replay the changes made on the
 `topic` branch since it diverged from `master` (i.e., `E`) until
 its current commit (`C`) on top of `master`, and record the result
 in a new commit along with the names of the two parent commits and
-a log message from the user describing the changes.
+a log message from the user describing the changes. Before the operation,
+`ORIG_HEAD` is set to the tip of the current branch (`C`).
 
 ------------
 	  A---B---C topic