Disable @python.Py3EnableCppAdapter for typedef resolving to primitiv…

…e and structs

Summary: Initial use cases seems like mostly containers, binary, and string. Let's restrict it.

Reviewed By: createdbysk

Differential Revision: D66511516

fbshipit-source-id: a10f50de4b23b779948078c048c397449993f951

thrift/annotation/python.thrift

@@ -97,5 +97,6 @@ struct UseCAPI {
 }
 
 /// Enables C++ Adapter for thrift-py3. It treats C++ Adapter on typedef as cpp.Type.
+/// It is only available for typedefs that resolve to binary, string, and container type.
 @scope.Typedef
 struct Py3EnableCppAdapter {}