adds comment explaining source of *Request names

Author: chalmerlowe

scripts/microgenerator/templates/partials/_method_with_request_builder.j2

@@ -2,6 +2,7 @@ def {{ method.name }}(
     self,
     {{ 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 #}
     request: Optional[{{ '.'.join(method.request_class_full_name.split('.')[-2:]) }}] = None,
     retry: OptionalRetry = DEFAULT_RETRY,
     timeout: Union[float, object] = DEFAULT_TIMEOUT,
@@ -40,4 +41,4 @@ def {{ method.name }}(
         retry=retry,
         timeout=timeout,
         metadata=metadata,
-    )
+    )