[css-values] Tighten up allowances for ignoring calc() on internal table widths and heights to only mixed-input cases. w3c#94

Author: fantasai

css-values/Overview.bs

@@ -1688,7 +1688,7 @@ Computed Value</h4>
 	0 and thus introduce computed-time division-by-zero. -->
 
 	Given the complexities of width and height calculations on table cells and table elements,
-	math expressions involving percentages for widths and heights on
+	math expressions mixing both percentages and lengths for widths and heights on
 	table columns, table column groups, table rows, table row groups, and table cells
 	in both auto and fixed layout tables
 	MAY be treated as if ''width/auto'' had been specified.