Add brief docs describing use of fixed service port with firewall

Minimal docs that should point users with firewall issues in the right direction.

Based on personal discovery and the explanation in Spotifyd#998 (comment).

docs/src/config/File.md

@@ -106,7 +106,10 @@ normalisation_pregain = -10
 # After the music playback has ended, start playing similar songs based on the previous tracks.
 autoplay = true
 
-# The port `spotifyd` uses to announce its service over the network.
+# The port at which `spotifyd` is going to offer its service over the network (TCP).
+# If not set, a random port > 1024 is used. For the service to be discoverable on the
+# local network via mDNS, both the mDNS port (5353 UDP) and the random or fixed
+# zeroconf port need to be allowed through any active firewall.
 zeroconf_port = 1234
 
 # The proxy `spotifyd` will use to connect to spotify.