follower read: add new option for tidb_replica_read (#2252) (#2283)

Signed-off-by: qupeng <qupeng@pingcap.com>

Co-authored-by: qupeng <qupeng@pingcap.com>
Co-authored-by: Shen Li <shenli3514@gmail.com>

reference/performance/follower-read.md

@@ -18,7 +18,7 @@ The Follower Read feature refers to using any follower replica of a Region to se
 
 ## Usage
 
-To enable TiDB's Follower Read feature, set the value of the `tidb_replica_read` session variable to `follower`:
+To enable TiDB's Follower Read feature, set the value of the `tidb_replica_read` session variable to `follower` or `leader-and-follower`:
 
 {{<copyable "sql">}}
 
@@ -34,6 +34,7 @@ This variable is used to set the data read mode expected by the current session.
 
 - When the value of `tidb_replica_read` is set to `leader` or an empty string, TiDB maintains its original behavior and sends all read operations to the leader replica to perform.
 - When the value of `tidb_replica_read` is set to `follower`, TiDB selects a follower replica of the Region to perform all read operations.
+- When the value of `tidb_replica_read` is set to `leader-and-follower`, TiDB can select any replicas to perform read operations.
 
 ## Implementation mechanism