Update next/image docs for relative parent with layout=fill. (vercel#26615)

vercel#18739 (reply in thread)

Author: leerob

docs/api-reference/next/image.md

@@ -102,7 +102,7 @@ When `responsive`, the image will scale the dimensions down for smaller
 viewports and scale up for larger viewports.
 
 When `fill`, the image will stretch both width and height to the dimensions of
-the parent element, usually paired with the [`objectFit`](#objectFit) property.
+the parent element, provided the parent element is relative. This is usually paired with the [`objectFit`](#objectFit) property.
 
 Try it out: