gh-117041: Add "-X gil" in the Python CLI help (GH-117042)

Author: serhiy-storchaka

Python/initconfig.c

@@ -195,6 +195,11 @@ The following implementation-specific options are available:\n\
 -X frozen_modules=[on|off]: whether to use frozen modules; the default is \"on\"\n\
          for installed Python and \"off\" for a local build;\n\
          also PYTHON_FROZEN_MODULES\n\
+"
+#ifdef Py_GIL_DISABLED
+"-X gil=[0|1]: enable (1) or disable (0) the GIL; also PYTHON_GIL\n"
+#endif
+"\
 -X importtime: show how long each import takes; also PYTHONPROFILEIMPORTTIME\n\
 -X int_max_str_digits=N: limit the size of int<->str conversions;\n\
          0 disables the limit; also PYTHONINTMAXSTRDIGITS\n\