"git submodule sync --recursive" documentation omission

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 appears to work, so the documentation should be
updated.

Tested with version 2.6.3.

    /ceder