[css-conditional-3] Add note clarifying that !important is not part of property grammar. w3c#5692

Author: fantasai

css-conditional-3/Overview.bs

@@ -660,6 +660,10 @@ partial namespace CSS {
       Note: No CSS escape or whitespace processing is performed on the property name,
       so <code>CSS.supports(" width", "5px")</code> will return <code>false</code>,
       as " width" isn't the name of any property due to the leading space.
+
+      Note: ''!important'' flags are not part of property grammars,
+      and will cause |value| to parse as invalid,
+      just as they would in the value argument to ''element.style.setProperty()''.
     </div>
 
 	<dt><code>supports(CSSOMString conditionText)</code>, returns <code>boolean</code>