[css-overflow-3] Add clarifying note on formating contexts

Author: frivoal

css-overflow-3/Overview.bs

@@ -1151,6 +1151,8 @@ Once it is sufficiently stable in this specification,
 
 	</dl>
 
+	Note: This property does **not** cause the box to [[establish an independent formatting context=].
+
 	<div class=example>
 		Given an article with one excessively long overflowing line,
 		and four more lines than can fit in the [=fragmentation direction=] as in the illustration bellow,