[Resize-Observer] Added definition links for boxes, removed snap box for now to understand use cases further before adding it in.

Author: Greg Whitworth

resize-observer-1/Overview.bs

@@ -128,10 +128,17 @@ It is modeled after {{MutationObserver}} and <a>IntersectionObserver</a>.
 
 <pre class="idl">
     enum ResizeObserverSizeOptions {
-        "border-box", "content-box", "scroll-box", "pixel-snapped-box"
+        "border-box", "content-box", "scroll-box"
     };
 </pre>
 
+In order to enable flexibility for authors to observe 
+more boxes than solely the {{content-box}} ResizeObserver takes one or more of the following enums to observe
+
+* {{border-box}}  : the {{border-box}} as defined in CSS2
+* {{content-box}} : the {{content-box}} as defined in CSS2
+* {{content-box}} : the {{scrollport}} of the {{scroll container}}
+
 <p class="issue">Is 'pixel-snapped-box' the best name?</p>
 
 <pre class="idl">