tag doc: move the description of --[no-]merged earlier

tag doc: move the description of --[no-]merged earlier

Move the documentation for the --merged & --no-merged options earlier
in the documentation, to sit along the other switches, and right next
to the similar --contains and --points-at switches.

It makes more sense to group the options together, not have some
options after the like of <tagname>, <object>, <format> etc.

This was originally put there when the --merged & --no-merged options
were introduced in 5242860f54 ("tag.c: implement '--merged' and
'--no-merged' options", 2015-09-10). It's not apparent from that
commit that the documentation is being placed apart from other
options, rather than along with them, so this was likely missed in the
initial review.

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