bpo-34231: PYTHONBREAKPOINT is not documented on python --help (GH-8475)

diff --git a/Modules/main.c b/Modules/main.c
index 31ebbce..1f1bf53 100644
--- a/Modules/main.c
+++ b/Modules/main.c
@@ -144,6 +144,8 @@
 "PYTHONCOERCECLOCALE: if this variable is set to 0, it disables the locale\n"
 "   coercion behavior. Use PYTHONCOERCECLOCALE=warn to request display of\n"
 "   locale coercion and locale compatibility warnings on stderr.\n"
+"PYTHONBREAKPOINT: if this variable is set to 0, it disables the default\n"
+"   debugger. It can be set to the callable of your debugger of choice.\n"
 "PYTHONDEVMODE: enable the development mode.\n";
 
 static void