Document the server_audit_loc_info variable

In frames of fix for MDEV-9106 new variable was added for Audit Plugin, server_audit_loc_info. It is initialized with zeros and is supposed to be empty string in MTR tests, as far as I can see.

I wonder what values are expected there while working with MySQL 5.7 and with older MySQL/MariaDB versions. Please, document this clearly in the code and/or in a separate KB article. I see nothing about it at https://mariadb.com/kb/en/mariadb/server_audit-system-variables/, while it seems to have non-empty values at times.