[css-values-4] Break cyclic unit dependency

Closes #2115

Author: frivoal

css-values-4/Overview.bs

@@ -1060,7 +1060,7 @@ Font-relative lengths: the ''em'', ''ex'', ''cap'', ''ch'', ''ic'', ''rem'', ''l
 	Or should we just wait for Houdini Custom Units?
 
 	When used in the value of the 'font-size' property
-	(or the 'line-height' property, for the ''lh'' and ''rlh'' units)
+	(and additionally for the ''lh'' and ''rlh'' units in the 'line-height' property)
 	on the element they refer to,
 	these units refer to the computed font metrics of the parent element
 	(or the computed font metrics corresponding to the initial values of the 'font' property,