Documentation/git-submodule: cleanup "add" section

Documentation/git-submodule: cleanup "add" section

The "add" section for 'git-submodule' is redundant in its
description and the short synopsis line. Fix it.

Remove the redundant mentioning of the 'repository' argument
being mandatory.

The text is hard to read because of back-references, so remove
those.

Replace the word "humanish" by "canonical" as that conveys better
what we do to guess the path.

While at it, quote all occurrences of '.gitmodules' as that is an
important file in the submodule context, also link to it on its
first mention.

Helped-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Kaartic Sivaraam <kaarticsivaraam91196@gmail.com>
Reviewed-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>