[css-grid] Clarify that <<flex>> cannot be represented in calc() expressions. #955

Author: fantasai

css-grid/Overview.bs

@@ -1675,6 +1675,8 @@ 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.
+
 	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>,
 	which is then divided among the flex-sized rows and columns in proportion to their <a>flex factor</a>.