bpo-36157:Document PyInterpreterState_Main() (GH-12238)



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
2 files changed