Added a note about columns.

Author: bert-github

css-template/Overview.src.html

@@ -1293,6 +1293,10 @@ <h3 id=columns>Interaction of 'grid' and 'columns'</h3>
 <pre>BODY {grid: "a @ b" "c c c"; columns: 2}</pre>
 
 <p>The @-slot will have two columns.
+
+<p class=note>Note that this is the same as setting columns on the
+default slot: 'BODY::slot(*) {columns: 2}' apart from differences in
+the specificity of the selector.
 </div>
 
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