[css-values-4][editorial] Add a note that combinators aren't associative. w3c#8017

Author: tabatkins

css-values-4/Overview.bs

@@ -229,6 +229,10 @@ Component Value Combinators</h3>
 	b || a || c
 	</pre>
 
+	Note: Combinators are <em>not</em> associative, so grouping is significant.
+	For example, ''a || b || c'' and ''a || [ b || c ]'' are distinct grammars:
+	the first allows a value like ''b a c'', but the second does not.
+
 <h3 id="component-multipliers">
 Component Value Multipliers</h3>