ssh signing: clarify trustlevel usage in docs

ssh signing: clarify trustlevel usage in docs

facca53ac added verification for ssh signatures but incorrectly
described the usage of gpg.minTrustLevel. While the verifications
trustlevel is stil set to fully or undefined depending on if the key is
known or not it has no effect on the verification result. Unknown keys
will always fail verification. This commit updates the docs to match
this behaviour.

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