add system variable back for tidb_enable_point_get_plan_cache.

[AilinKid]

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after fix of pingcap/tidb#60272, we add tidb_enable_point_get_cache back, so we just add the variable document back

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[LGTM Timeline notifier]

Timeline:

• 2025-03-31 09:45:28.956953773 +0000 UTC m=+1472022.641189870: ☑️ agreed by crazycs520.

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[lilin90]

/gemini review

[gemini-code-assist]

Code Review

The pull request reintroduces the tidb_enable_point_get_cache system variable documentation. The change seems straightforward, but I've added a comment to ensure the description is comprehensive.

Merge Readiness

The pull request appears ready for merging, pending resolution of the minor documentation suggestion. I am unable to directly approve the pull request, and recommend that others review and approve this code before merging.