last_plan_from_binding documented but not merged

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1. What is the URL/path of the document related to this issue?

See: https://docs.pingcap.com/tidb/stable/system-variables#last_plan_from_binding-new-in-v40

2. How would you like to improve it?

This references a feature which has not merged yet: pingcap/tidb#18017 -- if it doesn't merge, it should be removed from the docs to reduce confusion (it confused me!)

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This issue is stale because it has been open for 60 days with no activity. If no comment has been made and the lifecycle/stale label is not removed, this issue will be closed in 15 days.

[TomShawn]

@nullnotnil Can this issue be closed now?

Yes

Actually, I double checked the PR. It is in master branch only. I think because of the high rate of conflicts, it won't immediately be cherry picked. We should remove the docs about this for the stable docs.