gh-123672: Clarify the usage of PyGILState* for subinterpreters

Doc/c-api/init.rst

@@ -919,8 +919,10 @@ from a C thread is::
 Note that the ``PyGILState_*`` functions assume there is only one global
 interpreter (created automatically by :c:func:`Py_Initialize`).  Python
 supports the creation of additional interpreters (using
-:c:func:`Py_NewInterpreter`), but mixing multiple interpreters and the
-``PyGILState_*`` API is unsupported.
+:c:func:`Py_NewInterpreter`), but switching between interpreters via the
+``PyGILState_*`` API is unsupported. On the other hand, *creating*
+sub-interpreters (whether they have a per-interpreter GIL or not, in
+the sense of :pep:`684`) still require to hold the GIL.
 
 
 .. _fork-and-threads: