doc: mention that 'git submodule update' fetches missing commits
'git submodule update' will fetch new commits from the submodule remote
if the SHA-1 recorded in the superproject is not found. This was not
mentioned in the documentation.
Helped-by: Junio C Hamano <gitster@pobox.com> Helped-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>