help: correct usage & behavior of "git help --all"

Do the same for the "--all" option that I did for "--guides" in
9856ea6785c (help: correct usage & behavior of "git help --guides",
2021-09-22). I.e. we've documented it as ignoring non-option
arguments, let's have it error out instead.

As with other changes made in 62f035aee3f (Merge branch
'ab/help-config-vars', 2021-10-13) this is technically a change in
behavior, but in practice it's just a bug fix. We were ignoring this
before, but by erroring we can simplify our documentation and
synopsis, as well as avoid user confusion as they wonder what the
difference between e.g. "git help --all" and "git help --all status"
is (there wasn't any difference).

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

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