document submodule sync --recursive

The git-submodule(1) is inconsistent. In the synopsis, it says:

       git submodule [--quiet] sync [--recursive] [--] [<path>...]

The description of the sync does not mention --recursive, and the
description of --recursive says that it is only available for foreach,
update and status.

The option was introduced (82f49f2, Teach --recursive to submodule
sync, 2012-10-26) a while ago, so let's document it, too.

Reported-by: Per Cederqvist <cederp@opera.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-submodule.txt

@@ -237,6 +237,9 @@ sync::
 +
 "git submodule sync" synchronizes all submodules while
 "git submodule sync \-- A" synchronizes submodule "A" only.
++
+If `--recursive` is specified, this command will recurse into the
+registered submodules, and sync any nested submodules within.
 
 OPTIONS
 -------
@@ -364,7 +367,7 @@ the submodule itself.
 for linkgit:git-clone[1]'s `--reference` and `--shared` options carefully.
 
 --recursive::
-	This option is only valid for foreach, update and status commands.
+	This option is only valid for foreach, update, status and sync commands.
 	Traverse submodules recursively. The operation is performed not
 	only in the submodules of the current repo, but also
 	in any nested submodules inside those submodules (and so on).