PyImport_CreateModuleFromInitfunc: Doc & test update

[encukou]

tests:

• Test with a single-phase module as well.
• Ensure the module got initialized.
• Avoid internal API. (You probably want _call_with_frames_removed in the real world, but you can lose the _imp; spec_from_loader is re-exported and documented publicly)

docs:

• Note that the module is not initialized.
• Be explicit about errors.

The PyModule_Exec function (used in both tests and docs) is new for PEP-793, implemented in main but only documented in a draft PR as of right now (python#141197).

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