Update `Doing Your Own Relay Measurements`

Due to tpo/network-health/sbws#40190 (moved), @tornth tried to measure their own relays following the instructions at the bottom of https://support.torproject.org/#relay-operators_why-is-my-relay-slow, which link to tor-relays mailing list email from 2016 (https://lists.torproject.org/pipermail/tor-relays/2016-September/010173.html). They were getting this tor log:

[notice] Failed to find node for hop #1 of our path. Discarding this circuit.
[notice] Our circuit 0 (id: 5) died due to an invalid selected path, purpose Unlinked conflux circuit. This may be a torrc configuration issue, or a bug.

As @arma and @trinity-1686a pointed out, it isn't possible to use EntryNodes with non-guards, so we could update that section explaining how to configure tor for guards and non-guard relays.