[3.13] Document Py_AddPendingCall() change with subinterpreters in 3.12 (GH-139117) (GH-139119)

Document `Py_AddPendingCall()` change with subinterpreters in 3.12 (GH-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 GH-104813, this was undone, so Py_AddPendingCall() would always schedule for the main interpreter.
(cherry picked from commit 89ff88b)

Co-authored-by: Peter Bierma <[email protected]>

Author: miss-islington

Doc/c-api/init.rst

@@ -1944,6 +1944,10 @@ 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:
 
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