[css-values] Explain how to absolutize line-height

Closes w3c#1253

Author: frivoal

css-values-4/Overview.bs

@@ -1033,12 +1033,12 @@ Font-relative lengths: the ''em'', ''ex'', ''cap'', ''ch'', ''ic'', ''rem'', ''l
 		<dt><dfn id="lh" lt="lh">lh unit</dfn>
 		<dd>
 			Equal to the computed value of the 'line-height' property of the element on which it is used,
-			converted to an absolute length.
+			converted to an absolute length by measuring the height line box containing only the strut (See [[CSS2/visudet#strut]]) would have.
 
 		<dt><dfn id="rlh" lt="rlh">rlh unit</dfn>
 		<dd>
 			Equal to the computed value of 'line-height' property on the root element,
-			converted to an absolute length.
+			converted to an absolute length by measuring the height line box containing only the strut (See [[CSS2/visudet#strut]]) would have.
 			When specified on the 'font-size' or 'line-height' properties of the root element,
 			the ''rlh'' units refer to the properties' <em>initial value</em>.