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/cluster/hadoop/default.nix

@@ -7,33 +7,41 @@ with lib;
   options.services.hadoop = {
     coreSite = mkOption {
       default = {};
-      example = {
-        "fs.defaultFS" = "hdfs://localhost";
-      };
+      example = literalExample ''
+        {
+          "fs.defaultFS" = "hdfs://localhost";
+        }
+      '';
       description = "Hadoop core-site.xml definition";
     };
 
     hdfsSite = mkOption {
       default = {};
-      example = {
-        "dfs.nameservices" = "namenode1";
-      };
+      example = literalExample ''
+        {
+          "dfs.nameservices" = "namenode1";
+        }
+      '';
       description = "Hadoop hdfs-site.xml definition";
     };
 
     mapredSite = mkOption {
       default = {};
-      example = {
-        "mapreduce.map.cpu.vcores" = "1";
-      };
+      example = literalExample ''
+        {
+          "mapreduce.map.cpu.vcores" = "1";
+        }
+      '';
       description = "Hadoop mapred-site.xml definition";
     };
 
     yarnSite = mkOption {
       default = {};
-      example = {
-        "yarn.resourcemanager.ha.id" = "resourcemanager1";
-      };
+      example = literalExample ''
+        {
+          "yarn.resourcemanager.ha.id" = "resourcemanager1";
+        }
+      '';
       description = "Hadoop yarn-site.xml definition";
     };