[css-grid] Slightly rephrase the note.

Author: tabatkins

css-grid/Overview.bs

@@ -1675,7 +1675,9 @@ Flexible Lengths: the ''fr'' unit</h4>
 	A <dfn export>flexible length</dfn> or <dfn><<flex>></dfn> is a dimension with the ''fr'' unit,
 	which represents a fraction of the <a>free space</a> in the <a>grid container</a>.
 
-	Note: Unlike <<percentage>> values, <<flex>> values cannot be represented in or combined with other unit types in ''calc()'' expressions.
+	Note: <<flex>> values are not <<length>>s
+	(nor are they compatible with <<length>>s, like some <<percentage>> values),
+	so they cannot be represented in or combined with other unit types in ''calc()'' expressions.
 
 	The distribution of <a>free space</a> occurs after all non-flexible <a>track sizing functions</a> have reached their maximum.
 	The total size of such rows or columns is subtracted from the available space, yielding the <a>free space</a>,