nixos/zerotierone: document networks never left automatically

Removing a network from the joinNetworks list does not make the machine leave the network which is confusing.

This behavior is now clarified via the options description

nixos/modules/services/networking/zerotierone.nix

···
     example = [ "a8a2c3c10c1a68de" ];
     type = types.listOf types.str;
     description = lib.mdDoc ''
-      List of ZeroTier Network IDs to join on startup
+      List of ZeroTier Network IDs to join on startup.
+      Note that networks are only ever joined, but not automatically left after removing them from the list.
+      To remove networks, use the ZeroTier CLI: `zerotier-cli leave <network-id>`
     '';
   };