Add a guide on how to troubleshoot avahi (#14926)

Covers some common errors when using platform mdns on linux.

docs/guides/troubleshooting_avahi.md

@@ -0,0 +1,36 @@
+# Troubleshooting Avahi
+
+## Resetting the cache
+
+Avahi keeps a cache of old results. To reset the cache, kill the daemon. It will
+auto-restart.
+
+`sudo avahi-daemon --kill`
+
+## Stopping the daemon
+
+If you really want to stop the daemon, killing it is not sufficient because it
+will just restart. To stop it completely:
+
+```
+sudo systemctl mask avahi-daemon.socket
+sudo systemctl disable avahi-daemon
+sudo systemctl stop avahi-daemon
+```
+
+## Problem: Failed to create avahi group: Not permitted
+
+Avahi is not set up to publish records by default. This has to be explicitly
+allowed in the config file. In /etc/avahi/avahi-daemon.conf, add the following
+lines:
+
+```
+[publish]
+disable-user-service-publishing=no
+```
+
+Then restart the daemon
+
+```
+sudo systemctl restart avahi
+```