[css-text-decor-4] Mention property explicitely

See w3c#6828

Author: frivoal

css-text-decor-4/Overview.bs

@@ -668,7 +668,7 @@ Text Underline Offset: the 'text-underline-offset' property</h3>
 			<p>The UA chooses an appropriate offset for underlines.
 
 			However, this offset must be zero
-			if ''text-underline-position/from-font'' was specified
+			if the computed value of 'text-underline-position' is ''text-underline-position/from-font''
 			and the UA was able to extract an appropriate metric to use
 			from the font.