#33321 docs(defaults guard): name the Tunable helper forms the exact match does not see

Summary

A comment only, in tunable_source_defaults. It lists the forms of Tunable helper call that the exact match from #33161 does not see:

• a conformance or superclass in another file
• inheritance more than one level deep
• a nested generic clause
• a typealias of the helper type, or a local X.self
• a generic T.points
• a class func helper

The #33161 review found these forms. No Tunable helper in the tree is called in these ways today, and the lead decided to leave them.

This edit changes the guard file, so the defaults guard lists it as a change and needs a receipt.

Observed / Not run

python3 tests/test_defaults_guard.py passes (73 tests). prepush-gate.py is clean.

Changelog

none

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Summary by cubic

Adds a comment in tunable_source_defaults (scripts/ci/defaults_guard.py) documenting the Tunable helper call forms that the exact match from #33161 doesn't see: a conformance or superclass declared in another file, inheritance more than one level deep, a nested generic clause, a typealias or local X.self, a helper reached through a generic parameter (T.points), and a class func helper. No Tunable helper uses these forms today, so they remain unsupported. Editing the guard file also makes the defaults guard list it as a change requiring a receipt.