Document _PyStackRef

[colesbury]

We should document the behavior of _PyStackRef and associated functions. I think InternalDocs would be a good place for it.

cc @Fidget-Spinner @corona10

Linked PRs

• gh-124379: Document _PyStackRef #142321

[efimov-mikhail]

I propose to add tests that explain _PyStackRef behavior:
#139717

I think it could be useful as well as proper documentation in InternalDocs.