docs: annotate _keep_typed_in_additional_properties as a cross-SDK semantic marker

In Python, typed fields are already reachable via bracket notation through
_data_store, so no runtime behaviour change is required. The marker makes the
flag visible for introspection and aligns semantics with other language SDKs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: charlie-zhang109

.generator/src/generator/templates/model_generic.j2

@@ -75,6 +75,8 @@ class {{ name }}(ModelNormal):
     _nullable = True
 {%- endif %}
 {%- if model.get("x-keep-typed-in-additional-properties") %}
+    # Cross-SDK semantic marker. In Python, typed fields are already accessible via
+    # bracket notation (model["key"]) through _data_store, so no runtime change is needed.
     _keep_typed_in_additional_properties = True
 {%- endif %}