mktag doc: update to explain why to use this

mktag doc: update to explain why to use this

Change the mktag documentation to compare itself to the similar
"hash-object -t tag" command. Before this someone reading the
documentation wouldn't have much of an idea what the difference
was.

Let's allude to our own validation logic, and cross-link the "mktag"
and "hash-object" documentation to aid discover-ability. A follow-up
change to migrate "mktag" to use "fsck" validation will make the part
about validation logic clearer.

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