gpg docs: explain better use of ssh.defaultKeyCommand

gpg docs: explain better use of ssh.defaultKeyCommand

Using `ssh-add -L` for gpg.ssh.defaultKeyCommand is not a good
recommendation. It might switch keys depending on the order of known
keys and it only supports ssh-* and no ecdsa or other keys.
Clarify that we expect a literal key prefixed by `key::`, give valid
example use cases and refer to `user.signingKey` as the preferred
option.

Signed-off-by: Fabian Stelzer <fs@gigacodes.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>