Load Balancers: Monitoring states that "all other fields in the ServerDescription MUST remain unset." This includes the round-trip time (RTT).

Internally, MONGOC_RTT_UNSET is defined as -1; however, this is not mentioned in the documentation for mongoc_server_description_round_trip_time. A load balanced client may be the first scenarior where RTT may be unset, so this possible return value should be noted in the documentation.

Additionally, CDRIVER-4085 introduced a MONGOC_NO_SERVER_CONNECTION_ID constant. Its value is relevant to documentation for get_server_connection_id, so it'd make sense to publicize that constant as well.