git-submodule.txt: "--branch <branch>" option defaults to 'master'

This behavior is mentioned in gitmodules.txt but not in
git-submodule.txt so we copy the information over so that it is not
missed.

Also, add the missed argument to the -b/--branch option.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>