Clarify the bandwidth part of dir-spec
People keep asking about the precise meaning of relay bandwidths. We should make the spec clearer:
- there is a separate limit on inbound and outbound traffic
- traffic includes origin circuits and BEGINDIR requests
- let's check if traffic includes DirPort, I think it would have to
There may also be more feedback in #25854 (moved).
I'm tagging this fast-fix, because I can fix it fast, and it will save me time when I next explain it.