go/types: document that Underlying affects Named, Alias, TypeParam #65774
Description
Historically the Underlying method of go/types.Type used to strip off exactly one kind of type constructor: Named. Now, it removes Named, TypeParam (go1.18), and Alias (go1.22).
We should clarify the documentation of the interface method to call out this fact, and of the three concrete methods to state their behavior explicitly, and we should direct the reader to Unalias and other more discerning destructors.