wq-quick: document publicKey option

All exposed options should have documentation, and `nixosOptionDoc`
will give an error if it's missing.

modules/services/wg-quick.nix

@@ -35,6 +35,7 @@ let
       publicKey = mkOption {
         default = null;
         type = types.str;
+        description = "The public key for this peer.";
       };
     };
   };