system variables: update tidb_enable_noop_functions behavior

[unconsolable]

Signed-off-by: unconsolable chenzhipeng2012@gmail.com

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What is changed, added or deleted? (Required)

Document tidb_enable_noop_functions controls system variable sql_auto_is_null, which is introduced by pingcap/tidb#28247

Which TiDB version(s) do your changes apply to? (Required)

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• v5.2 (TiDB 5.2 versions)
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• v5.0 (TiDB 5.0 versions)
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• v2.1 (TiDB 2.1 versions)

What is the related PR or file link(s)?

• This PR is translated from: system variables: update tidb_enable_noop_functions behavior docs#6525
• Other reference link(s): executor, session, sessionctx/variable: add validate function for sysval sql_auto_is_null tidb#28247

Do your changes match any of the following descriptions?

• Delete files
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• Might cause conflicts after applied to another branch

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[TomShawn]

/merge

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

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Commit hash: 66b565b