bpo-42521: Add note about 'Python -d' only working on debug builds (GH-23607)

Python/initconfig.c

@@ -38,7 +38,8 @@ Options and arguments (and corresponding environment variables):\n\
          and comparing bytes/bytearray with str. (-bb: issue errors)\n\
 -B     : don't write .pyc files on import; also PYTHONDONTWRITEBYTECODE=x\n\
 -c cmd : program passed in as string (terminates option list)\n\
--d     : debug output from parser; also PYTHONDEBUG=x\n\
+-d     : turn on parser debugging output (for experts only, only works on\n\
+         debug builds); also PYTHONDEBUG=x\n\
 -E     : ignore PYTHON* environment variables (such as PYTHONPATH)\n\
 -h     : print this help message and exit (also --help)\n\
 ";