arrow

ARROW-6677: [FlightRPC][C++] Document Flight in C++

- Adds StatusDetail to the docs
- Fixes the Doxygen config to work with `ARROW_FLIGHT_EXPORT`
- Adds error codes to the format docs (though they're not part of the formal format)
- Touches up some docstrings and adds missing classes to the API docs
- Add a basic description of how to set up a Flight server/client in C++

Closes #5491 from lihalite/flight-docs and squashes the following commits:

294898302 <David Li> ARROW-6677:  Document Flight in C++

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>