[resize-observer] Fixed confusion around Element 'being rendered' - fixes w3c#3819

Author: Greg Whitworth

resize-observer-1/Overview.bs

@@ -76,7 +76,7 @@ ResizeObserver's notifications can be used to respond to changes in {{Element}}'
 
 * Observations will not be triggered by CSS transforms.
 
-* Observation will fire when observation starts if Element has display, and Element's size is not 0,0.
+* Observation will fire when observation starts if Element is [being rendered](https://html.spec.whatwg.org/#being-rendered), and Element's size is not 0,0.
 
 <div class="example">
   <pre highlight="html">