Document command line arguments.

muppet/__main__.py

@@ -13,8 +13,13 @@ def __main() -> None:
             prog='puppet-doc configure',
             description='Sets up puppet doc')
 
-    parser.add_argument('--env', action='store')
-    parser.add_argument('modules', nargs='*', type=pathlib.Path)
+    parser.add_argument('--env', action='store', help='''
+                        Path to a puppet `modules` directory.
+                        ''')
+    parser.add_argument('modules', nargs='*', type=pathlib.Path, help='''
+                        Any number of specific modules to generate documentation for.
+                        Mutually exclusive with --env.
+                        ''')
 
     args = parser.parse_args()