tag doc: reword --[no-]merged to talk about commits, not tips
Change the wording for the --merged and --no-merged options to talk
about "commits" instead of "tips".
This phrasing was copied from the "branch" documentation in commit
5242860f54 ("tag.c: implement '--merged' and '--no-merged' options",
2015-09-10). Talking about the "tip" is branch nomenclature, not
something usually associated with tags.
This phrasing might lead the reader to believe that these options
might find tags pointing to trees or blobs, let's instead be explicit
and only talk about commits.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>