[css-backgrounds-3] Add example of using gradients in background-image and add informative cross-reference to css-images-3.

Author: fantasai

css-backgrounds-3/Overview.bs

@@ -97,6 +97,9 @@ Value Definitions</h3>
   using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> from [[!CSS-VALUES-3]].
   Value types not defined in this specification are defined in CSS Values &amp; Units [[!CSS-VALUES-3]].
   Combination with other CSS modules may expand the definitions of these value types.
+  For example, combining with <a href="https://www.w3.org/TR/css-images/">CSS Images</a>
+  allows for using CSS gradients as 'background-image' or 'border-image' values.
+  [[CSS-IMAGES-3]]
 
   In addition to the property-specific values listed in their definitions,
   all properties defined in this specification
@@ -287,9 +290,10 @@ controls for image display.
 <div class="example">
 <p>Some examples specifying background images:</p>
 <pre>
-body { background-image: url("marble.svg") }
+html { background-image: url("marble.svg") }
 p { background-image: none }
 div { background-image: url(tl.png), url(tr.png) }
+main { background-image: radial-gradient(at bottom right, transparent, white); }
 </pre>
 </div>