Documentation: clarify that `-h` alone stands for `help`

We seem to be getting new users who get confused every 20 months or
so with this "-h consistently wants to give help, but the commands
to which `-h` may feel like a good short-form option want it to mean
something else." compromise.

Let's make sure that the readers know that `git cmd -h` (with no
other arguments) is a way to get usage text, even for commands like
ls-remote and grep.

Also extend the description that is already in gitcli.txt, as it is
clear that users still get confused with the current text.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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