Add Documentation for C-API Used by the _xxsubinterpreters Module

[ericsnowcurrently]

(from #101524 (comment))

C-API exposed by gh-107359:

• PyInterpreterID_Type
• PyInterpreterID_New()
• PyInterpreterID_LookUp()
• PyInterpreterState_GetIDObject()
• PyInterpreterState_RequireIDRef()
• atexit_datacallbackfunc
• PyUnstable_AtExit()
• PyUnstable_InterpreterState_GetMainModule()

Add tests too.

Linked PRs

• gh-107784 Document interpid C-API and new Unstable API for fetching __main__ of interpreter #112548

[ericsnowcurrently]

Also the _PyCrossInterpreterData API, though I expect PEP 554 will be addressing this for 3.13+.

(see #107068 (comment))

[tonybaloney]

@ericsnowcurrently you can assign this to me if you want, I've spent enough time looking at this code to document it and add tests.

[tonybaloney]

I've got the following two functions as well :

PyAPI_FUNC(int) _PyInterpreterState_RequiresIDRef(PyInterpreterState *);
PyAPI_FUNC(void) _PyInterpreterState_RequireIDRef(PyInterpreterState *, int);

You mentioned PyInterpreterState_RequireIDRef() but there isn't a public API function called that from what I can find (only the private one)

[tonybaloney]

I think Py_NewInterpreterFromConfig is new as well