nixos/treewide: remove boolean examples for options

They contain no useful information and increase the length of the
autogenerated options documentation.

See discussion in #18816.

nixos/modules/services/networking/firefox/sync-server.nix

@@ -33,7 +33,6 @@ in
       enable = mkOption {
         type = types.bool;
         default = false;
-        example = true;
         description = ''
           Whether to enable a Firefox Sync Server, this give the opportunity to
           Firefox users to store all synchronized data on their own server. To use this
@@ -78,7 +77,6 @@ in
       allowNewUsers = mkOption {
         type = types.bool;
         default = true;
-        example = false;
         description = ''
           Whether to allow new-user signups on the server. Only request by
           existing accounts will be honored.