[3.7] bpo-36157:Document PyInterpreterState_Main() (GH-12238) by miss-islington · Pull Request #12650

[3.7] bpo-36157:Document PyInterpreterState_Main() (GH-12238) by miss-islington · Pull Request #12650 · python/cpython

I have added documentation for `PyInterpreterState_Main()`.
 I chose to place it under Advanced Debugger Support together with similar functions like `PyInterpreterState_Head()`, `PyInterpreterState_Next(`), and `PyInterpreterState_ThreadHead()` .

https://bugs.python.org/issue36157
(cherry picked from commit 8c61739)

Co-authored-by: Joannah Nanjekye <33177550+nanjekyejoannah@users.noreply.github.com>