[css-backgrounds-4] Fix bg-clip grammar. Fixes #2916 properly.

Author: tabatkins

css-backgrounds-4/Overview.bs

@@ -130,16 +130,31 @@ Background Positioning Longhands: the 'background-position-x', 'background-posit
 <h3 id='background-clip'>
 Painting Area: the 'background-clip' property</h3>
 
-	<pre class="propdef partial">
+	<pre class="propdef">
 	Name: background-clip
-	New Values: text | border
+	Value: <<bg-clip>>#
+	Initial: border-box
+	Inherited: no
+	</pre>
+
+	Determines the <dfn export>background painting area</dfn>,
+	which determines the area within which the background is painted.
+	The syntax of the property is given with
+
+	<pre class=prod>
+	<dfn>&lt;bg-clip></dfn> = <<box>> | border | text
 	</pre>
 
 	Issue: Or should this be defining the <css>-webkit-background-clip</css> property,
 	saying that all the values are identical,
 	with this additional ''text'' value?
 
-	<dl dfn-type=value dfn-for=background-clip>
+	<dl dfn-type=value dfn-for="background-clip, <bg-clip>">
+		<dt><dfn><<box>></dfn>
+		<dd>
+			The background is painted within (clipped to)
+			the specified box of the element.
+
 		<dt><dfn>text</dfn>
 		<dd>
 			The background is painted within (clipped to)