Behavior of tidb_replica_read='learner' is inconsistent with the doc description

## Question

When a user sets `tidb_replica_read='learner'`, if there is no learner replica in the region, does TiDB just return an error or try another leader and follower replica?

Currently the [doc](https://docs.pingcap.com/tidb/stable/follower-read) said:

> When the value of tidb_replica_read is set to learner, TiDB reads data from the learner replica. If there is no learner replica in the Region, TiDB returns an error.

But the current behavior is:
1. Try the learner replica first
2. Try rest replicas(leader and follower)
3. If all replicas are not available, return an error.