modules: set docs.nixos.includeAllModules for frontend machines only

- not enabling it for just "useable" machines by default
- because rebuilds take way longer & manual gets rebuild on *every*
  module change

nix/nixos-modules/frontend/default.nix

@@ -35,6 +35,8 @@ in
       useXkbConfig = true;
     };
 
+    documentation.nixos.includeAllModules = true; # full manual aids NixOS dev
+
     environment = {
 
       pathsToLink = [

nix/nixos-modules/useable.nix

@@ -47,7 +47,7 @@ in
 
       nixos = {
         enable = lib.mkDefault true;
-        includeAllModules = lib.mkDefault true;
+        includeAllModules = lib.mkDefault false; # otherwise rebuilds full manual on each change of value
       };
 
     };