[Docs][FlightRPC] Update documentation to note that `FlightInfo::schema` may be null

### Describe the enhancement requested

Part of apache/arrow-java#167

The documentation should note that references to `FlightInfo::schema` may be null depending on the backend service implementing the Flight endpoint (e.g. there is a queue, or it is an expensive operation, or it happens later in query processing) and which message is used (i.e., when `pollFlightInfo` is used, the underlying engine may initially not populate this value; in `getFlightInfo`, the value must be populated).

### Component(s)

Documentation