public_member_api_docs ignores constructors in abstract interface classes

[Pante]

Describe the issue
public_member_api_docs does not flag constructors in abstract interface classes that are undocumented.

To Reproduce

abstract interface class Foo {
  factory Foo.a() = Bar;
  
  factory Foo.b() => Bar();

  Foo();
}

class Bar extends Foo {}

Expected behavior
The constructors should be flagged as undocumented, similar to how they are flagged in abstract & interface classes.

[pq]

This is related to #59210 but maybe we over-stepped in treating abstract interface classes as "effectively" private?

/cc @lrhn

[lrhn]

I think the #59210 discussion was correct in saying that generative constructors of abstract non-extensible classes can be treated as private, because they have no valid public uses.
That doesn't extend to factory constructors, which can be called and torn off just like any static method.
(Yet another difference between factory and generative constructors which make it breaking to change one to the other.)