git-commit.txt: clarify --patch mode with pathspec

git-commit.txt: clarify --patch mode with pathspec

How pathspec is used, with and without --interactive/--patch, is
different. But this is not clear from the document. These changes hint
the user to keep reading (to option #5) instead of stopping at #2 and
assuming --patch/--interactive behaves the same way.

And since all the options listed here always mention how the index is
involved (or not) in the final commit, add that bit for #5 as well. This
"on top of the index" is implied when you head over git-add(1), but if
you just go straight to the "Interactive mode" and not read what git-add
is for, you may miss it.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>