document submdule.$name.update=none option for gitmodules

document submdule.$name.update=none option for gitmodules

This option was not yet described in the gitmodules documentation. We
only described it in the 'git submodule' command documentation but
gitmodules is the more natural place to look.

A short reference in the 'git submodule' documentation should be
sufficient since the details can now be found in the documentation to
gitmodules.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>