check-docs: do not bother checking for legacy scripts' documentation

check-docs: do not bother checking for legacy scripts' documentation

In the recent years, there has been a big push to convert more and more
of Git's commands that are implemented as scripts to built-ins written
in pure, portable C, for robustness, speed and portability.

One strategy that served us well is to convert those scripts
incrementally, starting by renaming the scripts to
`git-legacy-<command>`, then introducing a built-in that does nothing
else at first than checking the config setting `<command>.useBuiltin`
(which defaults to `false` at the outset) and handing off to the legacy
script if so asked.

Obviously, those `git-legacy-<command>` commands share the documentation
with the built-in `git-<command>`, and are not intended to be called
directly anyway. So let's not try to ensure that they are documented
separately from their built-in versions.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>