Clarify that the RelayBandwidth* options exclude directory fetches by relays

Twice in the last few months, I have explained to relay operators that RelayBandwidthRate/Burst includes directory fetches from the relay by clients. But they do not include directory fetches by the relay (from authorities or other relays), because that is considered "client" activity.

We should clarify in the man page, because it clearly causes some confusion.