Allow options to be marked as "internal"

This means they're not for end users.  Currently they're filtered from
the manual, but we could include them in a separate section.

nixos/modules/misc/assertions.nix

@@ -13,6 +13,7 @@ in
   options = {
 
     assertions = mkOption {
+      internal = true;
       default = [];
       example = [ { assertion = false; message = "you can't enable this for that reason"; } ];
       merge = pkgs.lib.mergeListOption;