[docs] document --enable-incomplete-feature TypeForm, minimally but sufficiently (#20173)

Author: wyattscarpenter

docs/source/command_line.rst

@@ -1159,7 +1159,7 @@ format into the specified directory.
 Enabling incomplete/experimental features
 *****************************************
 
-.. option:: --enable-incomplete-feature {PreciseTupleTypes,InlineTypedDict}
+.. option:: --enable-incomplete-feature {PreciseTupleTypes,InlineTypedDict,TypeForm}
 
     Some features may require several mypy releases to implement, for example
     due to their complexity, potential for backwards incompatibility, or
@@ -1214,6 +1214,9 @@ List of currently incomplete/experimental features:
      def test_values() -> {"width": int, "description": str}:
          return {"width": 42, "description": "test"}
 
+* ``TypeForm``: this feature enables ``TypeForm``, as described in
+  `PEP 747 – Annotating Type Forms <https://peps.python.org/pep-0747/>_`.
+
 
 Miscellaneous
 *************