Documentation: tutorial: add information about "git help" at the beginning

Documentation: tutorial: add information about "git help" at the beginning

Talking about "git help" is useful because it has a few more
features (like when using it without arguments or with "-a") and
it may work on non unix like platforms.

Also add a few links to git-help(1) in "See also" sections.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>