Add note about safelisting

I added this to the upgrade guide previously but we should add it here too.

Author: thecrypticace

src/docs/functions-and-directives.mdx

@@ -259,7 +259,7 @@ Use the `@config` directive to load a legacy JavaScript-based configuration file
 @config "../../tailwind.config.js";
 ```
 
-The `corePlugins`, `safelist`, and `separator` options from the JavaScript-based config are not supported in v4.0.
+The `corePlugins`, `safelist`, and `separator` options from the JavaScript-based config are not supported in v4.0. To safelist utilities in v4 use [`@source inline()`](/docs/detecting-classes-in-source-files#safelisting-specific-utilities).
 
 <h3 id="plugin-directive">
   <a href="#plugin-directive">@plugin</a>