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

(cherry picked from commit d81e1ef)

Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Peter Bierma <zintensitydev@gmail.com>

Author: 2 people