stylix: simplify 'mkEnableTarget' documentation

Simplify the 'stylix.mkEnableTarget' documentation.

The following issue is to be resolved in
danth#400:

> Due to some targets not being enabled by default with
> 'stylix.mkEnableTarget', the documentation incorrectly generates
> 'Default: false' in some cases

stylix/target.nix

@@ -27,12 +27,8 @@ with lib;
       "styling for ${humanName}"
       // {
         default = config.stylix.autoEnable && autoEnable;
-
-        # We can't substitute the target name into this description because some
-        # don't make sense: "if the desktop background using Feh is installed"
-        defaultText = literalMD ''
-          `true` if `stylix.autoEnable == true` and the target is installed,
-          otherwise `false`.
-        '';
+      }
+      // optionalAttrs autoEnable {
+        defaultText = literalExpression "stylix.autoEnable";
       };
 }