Document the log level to use to create (email) alerts

As commented at #40119 (closed) and #40115 (closed), gabelmoo is often logging warnings about the its weight sum being more than the 50% of the consensus.

If a bwauth operator wish to receive (email) alerts, there might be quite a lot setting the log level to warning. Therefore recommend to bwauths to set the log level to error in such cases.