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.

1 files changed, 8 insertions, 6 deletions

diff --git a/nixos/modules/services/networking/syncthing.nix b/nixos/modules/services/networking/syncthing.nix
index 5b3eb6f04b4..e717d78feed 100644
--- a/nixos/modules/services/networking/syncthing.nix
+++ b/nixos/modules/services/networking/syncthing.nix
@@ -169,12 +169,14 @@ in {
           description = ''
             folders which should be shared by syncthing.
           '';
-          example = {
-            "/home/user/sync" = {
-              id = "syncme";
-              devices = [ "bigbox" ];
-            };
-          };
+          example = literalExample ''
+            {
+              "/home/user/sync" = {
+                id = "syncme";
+                devices = [ "bigbox" ];
+              };
+            }
+          '';
           type = types.attrsOf (types.submodule ({ name, ... }: {
             options = {