Document __pyx_capi__ #6484

Describe your issue

__pyx_capi__ seems to be used in other projects, so it became defacto API of cython[1][2]. Currently it is only mentioned in FAQ [3]. It seems to be important for the clients (see #3077 (comment)) and there is no definition of stability of this API. So I propose:

1. Document __pyx_capi__ since it is already an API used by other projects
2. Define the rules of the stability - e.g. we guarantee that it will be changed with deprecation period and migration strategy.

[1] https://github.com/ashwinvis/cython_capi
[2] https://github.com/numba/numba/blob/0f363d1b2dd19f2aa1a8cec5f0a99c3dd95512f8/numba/_helperlib.c#L570
[3] https://cython.readthedocs.io/en/latest/src/userguide/faq.html#how-do-i-compile-cython-with-subpackages