[css-multicol] update to spanner abspos text

Author: rachelandrew

css-multicol-1/Overview.bs

@@ -1104,7 +1104,9 @@ 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.
+	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.
 
 	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.  
@@ -1576,7 +1578,7 @@ This appendix is <em>informative</em>.
 <h3 id="changes-from-20191015">Changes from the <a href="https://www.w3.org/TR/2019/WD-css-multicol-1-20191015/">Working Draft (WD) of 15 October 2019</a></h3>
 
 <ul>
-	<li>Added the text "The spanner becomes the containing block for absolutely positioned boxes inside the spanner.". <a href="https://github.com/w3c/csswg-drafts/issues/5612#issuecomment-715369246">Resolved 23 Oct 2020</a>
+	<li>Added the text "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.". <a href="https://github.com/w3c/csswg-drafts/issues/5612#issuecomment-715369246">Resolved 23 Oct 2020</a>
 	</li>
 	<li>Adding the text "This property specifies whether content
 	in a multi-column line that does <em>not</em> immediately precede a spanner