Remind authority operators that bandwidth files should be written atomcally

Tor reads bandwidth files when it's voting, around 50 minutes past every hour, and 20 minutes past hours when the consensus has failed.

We should recommend that authority operators generate and transfer bandwidth files between 5-15 or 35-45 minutes outside of 15-25 and 45-55 minutes past the hour. The best place for this might be in dir-spec or the bandwidth file spec.