Meta: Clarify what IDL files need adding to idl_files.cmake

Author: https://github.com/Psychpsyo
Commit: https://github.com/LadybirdBrowser/ladybird/commit/49f65291dc5
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3520
Reviewed-by: https://github.com/AtkinsSJ ✅

Documentation/AddNewIDLFile.md

@@ -23,7 +23,7 @@ interface CSSRule {
 };
 ```
 
-3. Add a `libweb_js_bindings(HTML/HTMLDetailsElement)` call to [`LibWeb/idl_files.cmake`](../Libraries/LibWeb/idl_files.cmake)
+3. Add a `libweb_js_bindings(HTML/HTMLDetailsElement)` call to [`LibWeb/idl_files.cmake`](../Libraries/LibWeb/idl_files.cmake) (only for top-level interfaces)
 
 4. Forward declare the generated class in [`LibWeb/Forward.h`](../Libraries/LibWeb/Forward.h):
     - `HTMLDetailsElement` in its namespace.