add explicit public key naming for IdentitiesOnly usage

[analogist]

Add optional section of guide in case user has IdentitiesOnly yes turned on in ssh client configurations (common if other privacy/security-focused ssh guides were followed), since keys loaded in gpg-agent will then not auto-enumerate.

In such a case, it is not necessarily obvious that ssh -i [identity_file] or IdentityFile configurations can be pointed at a saved copy of the public key instead (since there should be no access to the private key).