事务相关的 next-gen 变量/配置变更

[ekexium]

Tracking issue

Update docs for TiDB X for these topcis:

• pessimistic-auto-commit default value
• tidb_txn_assertion_level default value

[hfxsd]

• pessimistic-auto-commit 的默认值为 false，请参考文档 https://docs.pingcap.com/zh/tidb/stable/tidb-configuration-file/#pessimistic-auto-commit
• tidb_txn_assertion_level 的默认值为 FAST，请参考文档 https://docs.pingcap.com/zh/tidb/stable/system-variables/#tidb_txn_assertion_level-%E4%BB%8E-v600-%E7%89%88%E6%9C%AC%E5%BC%80%E5%A7%8B%E5%BC%95%E5%85%A5

[ekexium]

This is a tracking issue of some changes needed for TiDB X. Please don't close it until corresponding TiDB X docs are stabilized.