doc lint: emit errors on STDERR

doc lint: emit errors on STDERR

Have all of the scripts invoked by "make check-docs" emit their output
on STDERR. This does not currently matter due to the way we're
invoking them, but will in a subsequent change. It's a good idea to do
this in any case for consistency with other tools we invoke.

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