Document tidb_force_priority = low_priority

References:
https://github.com/pingcap/tidb/issues/7524
https://github.com/pingcap/tidb/pull/7694
https://github.com/pingcap/tidb/pull/7777

It should be on this manual page: https://pingcap.com/docs/sql/server-command-option/

And possibly in a section under "Advanced Usage" recommending strategies to improve QoS of query time.