[css-values-4] Clarify/fix round(line-width) text. #13794

Author: tabatkins

css-values-4/Overview.bs

@@ -3347,11 +3347,15 @@ Stepped Value Functions: ''round()'', ''mod()'', and ''rem()''</h3>
 		:: If A is non-negative and B is omitted,
 			A is [=snapped as a border width=].
 
-			Otherwise, identical to ''<rounding-strategy>/nearest'',
-			except that if A is positive (non-zero)
-			and one of |lower B| or |upper B| is zero,
-			choose the non-zero one,
-			then [=snap as a border width=].
+			Otherwise, if A is non-negative and B is specified,
+			identical to ''<rounding-strategy>/nearest'',
+			except that if one of |lower B| or |upper B| is zero,
+			always choose the non-zero one.
+			Then [=snap as a border width=].
+
+			Otherwise (A is negative),
+			identical to ''<rounding-strategy>/nearest''.
+			(No snapping occurs.)
 	</dl>
 
 	If |lower B| would be zero,