adds comment regarding request_id_args

Author: chalmerlowe

scripts/microgenerator/templates/partials/_method_with_request_builder.j2

@@ -1,5 +1,6 @@
 def {{ method.name }}(
     self,
+    {# Assumes the primary resource ID is the last arg in request_id_args #}
     {{ method.request_id_args[-1] }}: Optional[str] = None,
     *,
     {# Assumes the request class is a nested type within the service, e.g., bigquery_v2.types.dataset.GetDatasetRequest #}