[css-multicol-1] Improve description of how spanners work.

Author: fantasai

css-multicol-1/Overview.bs

@@ -981,12 +981,13 @@ Spanning Columns</h2>
 		</dd>
 		<dt><dfn>all</dfn></dt>
 		<dd>
-			The element is taken [=out-of-flow=] 
-			and spans across all columns of the nearest multicol ancestor 
+			The element forces a column break and is taken [=out of flow=]
+			to span across all columns of the nearest multicol ancestor
 			in the same [=block formatting context=].
 			Content in the normal flow that appears before the element 
 			is automatically balanced across all columns 
-			in the immediately preceding [=multi-column line=] before the element appears.
+			in the immediately preceding [=multi-column line=] before the element appears,
+			and any subsequent content flows into a new [=multi-column line=] after the element.
 			The element establishes an [=independent formatting context=].
 
 			Note: Whether the element establishes a new <a>formatting context</a>
@@ -1069,8 +1070,8 @@ Spanning Columns</h2>
 	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.  
+	Although the spanner is taken [=out-of-flow=],
+	this does not affect the <a href="https://www.w3.org/TR/CSS2/zindex.html">painting order</a> [[!CSS21]] of the spanning element.
 
 	<div class="example">
 		In this example, an <code>h2</code> element has been added to the sample document after the sixth sentence