Document pam_ssh

The KnowledgeBase mentions that a pam_ssh module exists, but nothing more than that:

https://mariadb.com/kb/en/authentication-plugin-pam/#pam_ssh

This question shows that someone had troubles at using it, and could not find any help on SO:

https://dba.stackexchange.com/questions/334233/trying-to-authenticate-the-slurm-user-via-keys-instead-of-password-using-the-pam