Add missed description (#315)

Author: Jeremy-Walton

src/stories/Recipes/Layout/Layout.mdx

@@ -90,6 +90,8 @@ If you need a login page, use the following layout playground:
 
 ## Legacy Layout
 
+This is the legacy layout that is maintained for backwards compatibility. New projects should not use these classes and instead, should use the `op-page` layout above.
+
 ### Playground
 
 <Canvas of={LayoutStories.Basic} />