Adding a note in response to the TAG review.

Author: rachelandrew

css-multicol-2/Overview.bs

@@ -936,6 +936,11 @@ The number and size of columns</h2>
 	New rows repeat until all content is displayed.
 	</div>
 
+	Note: The reading flow of a 2-dimensional multiple-column layout follows along the row,
+	reading each column in row 1 in turn, then moving onto row 2, and so on.
+	Authors should ensure that it's clear to a reader how the content flows,
+	particularly in situations where multiple rows are displayed on screen.
+
 	Issue(11754): The issue relating to the addition of this property is still open due to debate as to whether it's required.
 
 	Issue(11754): What happens if you set a height on the multicol container, 'column-wrap: wrap' but no 'column-height'?