Documentation: describe 'submodule update --remote' use case

Documentation: describe 'submodule update --remote' use case

Make it clear that there is no implicit floating going on; --remote
lets you explicitly integrate the upstream branch in your current
HEAD (just like running 'git pull' in the submodule).  The only
distinction with the current 'git pull' is the config location and
setting used for the upstream branch, which is hopefully clear now.

Signed-off-by: W. Trevor King <wking@tremily.us>
Signed-off-by: Junio C Hamano <gitster@pobox.com>