bpo-38816: Add notes in the C-API docs about fork in subinterpreters. (GH-17176)


The C-API docs are a bit sparse on the interplay between C `fork()` and the CPython runtime.  This change adds some more information on the subject.

https://bugs.python.org/issue38816
(cherry picked from commit 73cdb0c6b2c3861e034004cdc57be5e726876078)

Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>
3 files changed