Document how to access trailing_metadata from ActiveCall::Operation on streaming responses

Currently the underlying `GRPC::ActiveCall::Operation` is exposed to users directly for every method call and it is the only way to access some gRPC properties, like the `trailing_metadata`.

This could be confusing because it is low-level and access to the these properties (i.e metadata) depends on the timing of the call. For example, `trailing_metadata` is not available immediately for a streaming call but it is for unary calls (see #239).

Do we need a higher level API for this?