doc: git-tag: stop focusing on GPG signed tags

It looks like the documentation of `git tag` is focused a bit too
much on GPG signed tags.

This starts with the "NAME" section where the command is described
with:

"Create, list, delete or verify a tag object signed with GPG"

while for example `git branch` is described with simply:

"List, create, or delete branches"

This could give the false impression that `git tag` only works with
tag objects, not with lightweight tags, and that tag objects are
always GPG signed.

In the "DESCRIPTION" section, it looks like only "GnuPG signed tag
objects" can be created by the `-s` and `-u <key-id>` options, and it
seems `gpg.program` can only specify a "custom GnuPG binary".

This goes on in the "OPTIONS" section too, especially about the `-s`
and `-u <key-id>` options.

The "CONFIGURATION" section also doesn't talk about how to configure
the command to work with X.509 and SSH signatures.

Let's rework all that to make sure users have a more accurate and
balanced view of what the command can do.

Helped-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>