git-tag.txt: list all modes in the description

git-tag.txt: list all modes in the description

Currently, the description sounds as if it applied always, but most of
its content is true in "create tag mode" only.

Make this clearer by listing all modes upfront.

Also, sneak in some linguistic improvements and make it clearer that
lightweight tags are "created" because "written" may be misread as
"are output".

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>