Improve documentation and help text for MariaDB Backup's --plugin-dir option

In MDEV-13400, I mentioned that MariaDB backup ignores the --plugin-dir command-line option during the backup stage. Instead, the tool queries the server for its value, and it always uses that value. It sounds like we do not want to change this.

I believe that the --plugin-dir command-line option is read during the prepare stage.

We should probably update the --help text of the mariabackup tool to specify that the --plugin-dir option is only valid during the prepare stage. We should probably add a similar note to the documentation:

https://mariadb.com/kb/en/the-mariadb-library/mariabackup-options/

Should the utility also warn the user if they specify the --plugin-dir command-line option during the backup stage?