[css-multicol-1] Clarify containing block of spanner and its contents. #5612

* Spanners always form a containing block for their in-flow contents;
  don't imply that this causes them to trap abspos as well.
* Clarify that the containing block of the spanner is the multicol container.

Author: fantasai

css-multicol-1/Overview.bs

@@ -1063,9 +1063,11 @@ Spanning Columns</h2>
 	An element that spans more than one column is called a <dfn export>spanning element</dfn> 
 	and the box it creates is called a <dfn export>spanner</dfn>.
 
-	The spanner becomes the [=containing block=] for [=absolutely positioned boxes=] inside the spanner 
-	where the spanning element establishes a [=containing block=], 
-	otherwise the containing block chain goes to the multicol container.
+	The [=containing block=] of the [=spanner=] is the [=multicol container=] itself.
+	Consequently, in cases where the spanner itself does not establish
+	a [=containing block=] for [=absolutely positioned boxes=] inside the spanner,
+	their [=containing block chain=] skips directly to the [=multicol container=]
+	(skipping any ancestors between the [=spanner=] and the [=multicol container=]).
 
 	A spanning element is taken [=out-of-flow=], leaving a forced break. 
 	This does not affect the <a href="https://www.w3.org/TR/CSS2/zindex.html">painting order</a> [[!CSS21]] of the spanning element.