Document new mariadb-import options from MDEV-33627

There is a new option --dir, to restore from mariadb-dump --dir, as well as
several options to facilitate partial restore (--table, can be used multiple times, --database, --ignore-table, --ignore-database)

There is some explanation of what new options do in this commit comment
https://github.com/MariaDB/server/commit/9e25d6f0cc3ce965ebb7b861813b4a2f7aedf04c