[3.14] pythongh-138189: Document type slots, and other constants, as part of Limited API (pythonGH-138190) (pythonGH-141131)

(cherry picked from commit d81e1ef)

Co-authored-by: Petr Viktorin <[email protected]>
Co-authored-by: Peter Bierma <[email protected]>

Author: 3 people