add doc for @@tidb_enable_noop_variables

[tangenta]

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• Other reference link(s): *: add support for disabling noop variables tidb#35496

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[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn

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[djshow832]

Rest LGTM

[TomShawn]

LGTM

[TomShawn]

/remove-status LGT1
/status LGT2

[TomShawn]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 1d8885e