Add a comment to _Py_RestoreSignals() (GH-18792)

subprocess _posix_spawn() should stay in sync with _Py_RestoreSignals().

Python/pylifecycle.c

@@ -2374,6 +2374,8 @@ init_signals(PyThreadState *tstate)
  * All of the code in this function must only use async-signal-safe functions,
  * listed at `man 7 signal` or
  * http://www.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_04.html.
+ *
+ * If this function is updated, update also _posix_spawn() of subprocess.py.
  */
 void
 _Py_RestoreSignals(void)