gh-116991: Improve pygen --help for python subparser (#116992)

Author: sobolevn

Tools/peg_generator/pegen/__main__.py

@@ -107,7 +107,10 @@ def generate_python_code(
     help="Suppress code emission for rule actions",
 )
 
-python_parser = subparsers.add_parser("python", help="Generate Python code")
+python_parser = subparsers.add_parser(
+    "python",
+    help="Generate Python code, needs grammar definition with Python actions",
+)
 python_parser.set_defaults(func=generate_python_code)
 python_parser.add_argument("grammar_filename", help="Grammar description")
 python_parser.add_argument(