document submodule sync --recursive

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 (82f49f294c, 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>