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
(cherry picked from commit 8c61739defd88c7f79e86537886c33745843ce01)

Co-authored-by: Joannah Nanjekye <33177550+nanjekyejoannah@users.noreply.github.com>
diff --git a/Doc/c-api/init.rst b/Doc/c-api/init.rst
index 2cdc052..a88873b 100644
--- a/Doc/c-api/init.rst
+++ b/Doc/c-api/init.rst
@@ -1383,6 +1383,11 @@
    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