git help doc: use "<doc>" instead of "<guide>"

git help doc: use "<doc>" instead of "<guide>"

Replace the use of "<guide>" originally introduced (as "GUIDE") in
a133737b809 (doc: include --guide option description for "git help",
2013-04-02) with the more generic "<doc>". The "<doc>" placeholder is
more generic, and one we'll be able to use as we introduce new
documentation categories.

Let's also add "<doc>" to the "git help -h" output, when it was made
to use parse_option() in in 41eb33bd0cb (help: use parseopt,
2008-02-24).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>