Add missing 'background-image' to list of properties reset by 'background' shorthand.

Author: fantasai

css3-background/Overview.html

@@ -1809,6 +1809,8 @@ <h3 id=the-background><span class=secno>3.10. </span>Backgrounds Shorthand:
    style sheet. The number of comma-separated items defines the number of
    background layers. Given a valid declaration, for each layer the shorthand
    first sets the corresponding layer of each of &lsquo;<a
+   href="#background-image"><code
+   class=property>background-image</code></a>&rsquo;, &lsquo;<a
    href="#background-position"><code
    class=property>background-position</code></a>&rsquo;, &lsquo;<a
    href="#background-size"><code

css3-background/Overview.src.html

@@ -1218,6 +1218,7 @@ <h3 id="the-background">Backgrounds Shorthand: the 'background' property</h3>
 place in the style sheet. The number of comma-separated items defines
 the number of background layers. Given a valid declaration, for each layer
 the shorthand first sets the corresponding layer of each of
+'background-image',
 'background-position',
 'background-size',
 'background-repeat',