Improve relay overload documentation

We got some feedback in our forum on how to improve our overload page. In particular:

* Tuning `sysctl` as described now is not permanent and will be lost upon restart. You need to add the configuration to `/etc/sysctl.conf` or to a file in `/etc/sysctl.d/` to make it permanent.
* The DNS timeout is applicable to exit nodes middle or guard relays do not really need to worry about DNS.
* The `MetricsPort` was the most difficult to understand. It took me quite a bit to find an example on how to add the configuration to the `/etc/tor/torrc` file, but after that, it is not clear at all to me how to get the debug information I need. Ultimately, I found a [grafana dashboard](https://gitlab.com/TheHolm/tor-node-grafana-dashboard) that could make use of that info and I installed it (which meant figuring out how to install prometheus and grafana) but the information I am getting from there are not really helpful or at least I don't understand the statistics I am seeing.

The third item is tackled in #265 (closed) while this ticket will be focused on the first two.