"TiDB uses optimistic concurrency control by default" does not look like valid since TiDB 3.0.8 `tidb_txn_mode` changed default value to `pessimistic`

[yahonda]

File: /master/optimistic-transaction.md

Current description

says

TiDB uses optimistic concurrency control by default

Expected one

Since TiDB 3.0.8, TiDB uses pessimistic concurrency control by default. The document linked above should follow the default behavior.

https://docs.pingcap.com/tidb/stable/release-3.0.8/

Update the default value of the tidb_txn_mode variable from "" to "pessimistic" when a new cluster is created #14171

[yahonda]

"TiDB uses optimistic concurrency control by default whereas MySQL applies pessimistic concurrency control. " statement was added via #2088