[css-multicol-1] Clarify that 'column-fill' does not affect columns immediately preceding a spanner per WG resolution. w3c#4689 <https://lists.w3.org/Archives/Public/www-style/2020May/0002.html>

Author: fantasai

css-multicol-1/Overview.bs

@@ -1277,6 +1277,9 @@ Filling Columns</h2>
 	Animation type: discrete
 	</pre>
 
+	This property specifies whether content
+	in a [=multi-column line=] that does <em>not</em> immediately preced a [=spanner=]
+	is balanced across columns or not.
 	The values are:
 
 	<dl dfn-type=value dfn-for=column-fill>