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

Doc/c-api/init.rst

@@ -1395,6 +1395,11 @@ These functions are only intended to be used by advanced debugging tools.
    Return the interpreter state object at the head of the list of all such objects.
 
 
+.. c:function:: PyInterpreterState* PyInterpreterState_Main()
+
+   Return the main interpreter state object.
+
+
 .. c:function:: PyInterpreterState* PyInterpreterState_Next(PyInterpreterState *interp)
 
    Return the next interpreter state object after *interp* from the list of all

Misc/NEWS.d/next/Documentation/2019-03-08-15-39-47.bpo-36157.nF1pP1.rst

+Added Documention for  PyInterpreterState_Main().
\ No newline at end of file