doc txt & -h consistency: add "-z" to cat-file "-h"

doc txt & -h consistency: add "-z" to cat-file "-h"

Fix a bug in db9d67f2e9c (builtin/cat-file.c: support NUL-delimited
input with `-z`, 2022-07-22), before that change the SYNOPSIS and "-h"
output were the same, but not afterwards.

That change followed a similar earlier divergence in
473fa2df08d (Documentation: add --batch-command to cat-file synopsis,
2022-04-07). Subsequent commits will fix this sort of thing more
systematically, but let's fix this one as a one-off.

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