[css-color-4] Add production rule for <opacity-value>

Author: cdoublev

css-color-4/Overview.bs

@@ -657,6 +657,11 @@ Transparency: the 'opacity' property</h3>
 
 	Opacity in CSS is represented using the <<opacity-value>> syntax,
 	for example in the 'opacity' property.
+
+	<pre class='prod'>
+		<<opacity-value>> = <<number>> | <<percentage>>
+	</pre>
+
 	Represented as a <<number>>, the useful range of the value is ''0''
 	(representing full transparency)
 	to ''1''