Make image-set() only require a single argument, to avoid editing hazards.

Author: tabatkins

css4-images/Overview.html

@@ -831,7 +831,7 @@ <h3 id=image-set-notation><span class=secno>4.3. </span> Resolution
 <!--
 	--><dfn
    id=image-set-type>&lt;image-set></dfn> = image-set( [ <a
-   href="#image-set-decl-type"><i>&lt;image-set-decl></i></a>, ]+ [ <a
+   href="#image-set-decl-type"><i>&lt;image-set-decl></i></a>, ]* [ <a
    href="#image-set-decl-type"><i>&lt;image-set-decl></i></a> | &lt;color>] )
 <!--
 	--><dfn

css4-images/Overview.src.html

@@ -552,7 +552,7 @@ <h3 id='image-set-notation'>
 		The syntax for ''image-set()'' is:
 
 	<pre class='prod'>&#xa;<!--
-	--><dfn id='image-set-type'>&lt;image-set></dfn> = image-set( [ <i>&lt;image-set-decl></i>, ]+ [ <i>&lt;image-set-decl></i> | &lt;color>] )&#xa;<!--
+	--><dfn id='image-set-type'>&lt;image-set></dfn> = image-set( [ <i>&lt;image-set-decl></i>, ]* [ <i>&lt;image-set-decl></i> | &lt;color>] )&#xa;<!--
 	--><dfn id='image-set-decl-type'>&lt;image-set-decl></dfn> = [ &lt;url> | &lt;string> ] <i>&lt;resolution></i></pre>
 
 	<p>