[dnsdist] documentation for "showDOHFrontends()" output

[Christoph]

> most of the columns in the output of "showDOHFrontends()" are
> self-explanatory, but it would be interesting to know the details behind
>  "Bad" and "Errors".
> 
> Are these columns documented somewhere?
> 
> https://dnsdist.org/reference/config.html#showDOHFrontends

The "Errors" counter appears to match the amount of HTTP requests
getting answered with:

500 Internal Server Error

The "Bad" column is still unknown to me but it is not
related to HTTP 400 or 408 response codes.


related feature request:
https://github.com/PowerDNS/pdns/issues/7898