nixos/treewide: Fix incorrectly rendered examples

Many options define their example to be a Nix value without using
literalExample. This sometimes gets rendered incorrectly in the manual,
causing confusion like in https://github.com/NixOS/nixpkgs/issues/25516

This fixes it by using literalExample for such options. The list of
option to fix was determined with this expression:

  let
    nixos = import ./nixos { configuration = {}; };
    lib = import ./lib;
    valid = d: {
      # escapeNixIdentifier from https://github.com/NixOS/nixpkgs/pull/82461
      set = lib.all (n: lib.strings.escapeNixIdentifier n == n) (lib.attrNames d) && lib.all (v: valid v) (lib.attrValues d);
      list = lib.all (v: valid v) d;
    }.${builtins.typeOf d} or true;

    optionList = lib.optionAttrSetToDocList nixos.options;

  in map (opt: {
    file = lib.elemAt opt.declarations 0;
    loc = lib.options.showOption opt.loc;
  }) (lib.filter (opt: if opt ? example then ! valid opt.example else false) optionList)

which when evaluated will output all options that use a Nix identifier
that would need escaping as an attribute name.

nixos/modules/services/networking/3proxy.nix

@@ -334,10 +334,12 @@ in {
           nsrecord = mkOption {
             type = types.attrsOf types.str;
             default = { };
-            example = {
-              "files.local" = "192.168.1.12";
-              "site.local" = "192.168.1.43";
-            };
+            example = literalExample ''
+              {
+                "files.local" = "192.168.1.12";
+                "site.local" = "192.168.1.43";
+              }
+            '';
             description = "Adds static nsrecords.";
           };
         };