signature-format.txt: note SSH and X.509 signature delimiters

This document only explains PGP signatures, but Git now supports X.509
signatures as of 1e7adb9756 (gpg-interface: introduce new signature
format "x509" using gpgsm, 2018-07-17), and SSH signatures as of
29b315778e (ssh signing: add ssh key format and signing code,
2021-09-10).

Additionally, explain that these signature formats are controlled
`gpg.format`, linking to its documentation, and explain in said
`gpg.format` documentation that the underlying signature format is
documented in signature-format.txt.

Signed-off-by: Gwyneth Morgan <gwymor@tilde.club>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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