doc txt & -h consistency: add or fix optional "--" syntax

doc txt & -h consistency: add or fix optional "--" syntax

Add the "[--]" for those cases where the *.txt and -h were
inconsistent, or where we incorrectly stated in one but not the other
that the "--" was mandatory.

In the case of "rev-list" both sides were wrong, as we we don't
require one or more paths if "--" is used, e.g. this is OK:

	git rev-list HEAD --

That part of this change is not a "doc txt & -h consistency" change,
as we're changing both versions, doing so here makes both sides
consistent.

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