Document Py_AddPendingCall() change with subinterpreters in 3.12 (pythonGH-139117)

Prior to 3.9, Py_AddPendingCall() would always run pending calls in the main interpreter, but then each interpreter got their own ceval state, and they were scheduled for any interpreter. In pythonGH-104813, this was undone, so Py_AddPendingCall() would always schedule for the main interpreter.
(cherry picked from commit 89ff88b)

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>

Author: ZeroIntensity

Doc/c-api/init.rst

@@ -1999,6 +1999,10 @@ pointer and a void pointer argument.
       called from the main interpreter. Each subinterpreter now has its own
       list of scheduled calls.
 
+   .. versionchanged:: 3.12
+      This function now always schedules *func* to be run in the main
+      interpreter.
+
 .. _profiling:
 
 Profiling and Tracing