Add missing 'type', 'defaultText' and 'literalExample' in module definitions

- add missing types in module definitions
- add missing 'defaultText' in module definitions
- wrap example with 'literalExample' where necessary in module definitions

nixos/modules/tasks/network-interfaces.nix

@@ -355,6 +355,7 @@ in
     };
 
     networking.nameservers = mkOption {
+      type = types.listOf types.str;
       default = [];
       example = ["130.161.158.4" "130.161.33.17"];
       description = ''
@@ -390,6 +391,7 @@ in
     };
 
     networking.localCommands = mkOption {
+      type = types.str;
       default = "";
       example = "text=anything; echo You can put $text here.";
       description = ''