[css-values-4] Clarify that simplification happens at used-value time as well, and always reduces the math function to a numeric value.

Author: tabatkins

css-values-4/Overview.bs

@@ -3490,7 +3490,7 @@ Computed Value</h3>
 
 	The [=computed value=] of a [=math function=]
 	is its [=calculation tree=] [=simplified=],
-	using all the information available at [=computed-value=] time.
+	using all the information available at [=computed value=] time.
 	(Such as the ''em'' to ''px'' ratio,
 	how to resolve percentages in some properties,
 	etc.)
@@ -3503,6 +3503,10 @@ Computed Value</h3>
 	(e.g. <a href="https://www.w3.org/TR/CSS21/visudet.html#the-height-property">the <css>height</css> property</a>),
 	they apply whenever a [=math function=] contains percentages.
 
+	The [=calculation tree=] is again simplified at [=used value=] time;
+	with [=used value=] time information,
+	a [=math function=] always simplifies down to a single numeric value.
+
 	<div class='example'>
 		For example,
 		whereas 'font-size' computes percentage values at <a>computed value</a> time