Clarify effect of column-rule-style: hidden on column-rule-width. #11090 (#11262)

Author: kbabbitt

css-multicol-1/Overview.bs

@@ -1166,7 +1166,7 @@ Stacking Context</h3>
 	parsing/column-rule-style-invalid.html
 	</wpt>
 
-	The ''border-style/none'' value forces the computed value of 'column-rule-width' to be ''0''.
+	The ''border-style/none'' and ''hidden'' values force the computed value of 'column-rule-width' to be ''0''.
 
 
 <h3 id='crw'>The Width Of Column Rules: the 'column-rule-width' property</h3>