planner: update doc for Instance plan cache

[qw4990]

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[songrijie]

Suggested change

	TiDB supports both Session Plan Cache and Instance Plan Cache. Session Plan Cache maintains cache for each session, plans can't be shared across different sessions, while Instance Plan Cache maintains one single cache in the instance for all sessions.
	TiDB supports both Session Plan Cache and Instance Plan Cache. Session Plan Cache maintains cache for each session, while Instance Plan Cache maintains one single cache in each instance which can be shared across all sessions in this instance.

[songrijie]

Suggested change

	> Instance Plan Cache only supported after v8.4 and it is an experimental feature.
	> Instance Plan Cache is experimental feature.

[songrijie]

Suggested change

	And for Instance Plan Cache, TiDB provides 2 system views (after v8.5) `information_schema.tidb_plan_cache` and `information_schema.cluster_tidb_plan_cache` to see the internal information of the Instance Plan Cache of the current instance and the whole cluster:
	In v8.5.0 and higher, two system tables were introduced to observe the entries in Instance Plan Cache:
	* `information_schema.tidb_plan_cache`: The entries of instance plan cache in this particular TiDB instance.
	* `information_schema.cluster_tidb_plan_cache`: The entries of instance plan cache from all available TiDB instances.