security/openssh-portable: fix docs when built without PAM support

The defaults documented in sshd_config and sshd_config.5 are incorrect
if OpenSSH was built without PAM support and can be misleading to the
user whether or not password authentication is enabled.

• Moved PAM specific changes out of patch-sshd_config and into extra-patch-pam-sshd_config
• sshd_config.5 PasswordAuthentication: added a new line before the note to make it easier to read.
• sshd_config.5 UsePAM: noted the default value depends on whether sshd was built with or without PAM support.

PR: 261342