[css-values-4] Call out the reuse of keywords between round() and block-step-size.

Author: tabatkins

css-values-4/Overview.bs

@@ -2647,6 +2647,13 @@ Rounding Values: ''round()''</h3>
 	is equivalent to CSS's ''round(nearest, ...)'',
 	or just ''round(...)''.
 
+	Note: The <<rounding-strategy>> keywords
+	are the same as the keywords in 'block-step-size'
+	and have the same behavior.
+	('block-step-size' just lacks ''to-zero'';
+	since block sizes are always non-negative,
+	''to-zero'' and ''down'' would be identical.)
+
 
 <h4 id=round-infinities>
 Argument Ranges</h4>