[css-paint-api] Add a note pointing to the css images concrete object size examples.

See issue w3c#417.

Author: bfgeek

css-paint-api/Overview.bs

@@ -464,6 +464,9 @@ Note: In a future version of the spec, the author could have the ability to spec
 The {{PaintSize}} object represents the size of the image that the author should draw. This is
 the <a>concrete object size</a> given by the user agent.
 
+Note: See [[css-images-3#object-sizing-examples]] for examples on how the <a>concrete object
+    size</a> is calculated.
+
 <pre class='idl'>
 [Exposed=PaintWorklet]
 interface PaintSize {