[css-sizing-3] Replace section consisting of two notes with non-normative section.

Author: fantasai

css-sizing-3/Overview.bs

@@ -358,20 +358,20 @@ New Keywords for 'width' and 'height'</h3>
 	these should be interpreted as meaning <a>behaves as auto</a>,
 	and reported to the CSSWG for updating.
 
-<h3 id='the-contain-floats-value'>
-Containing or Excluding Floats</h3>
-
-	Note: To ensure that a container sizes itself to contain any descendant floats,
-	make sure it's a <a>formatting context</a> root.
-	For some layout modes, such as Grid and Flexbox,
-	this is true automatically.
-	For Block layout, this means using ''display: flow-root;''.
-
-	Note: To size an element such that it avoids overlapping sibling floats,
-	make sure it's a <a>formatting context</a> root.
-	For some layout modes, such as Grid and Flexbox,
-	this is true automatically.
-	For Block layout, this means using ''display: flow-root;''.
+<h4 id='the-contain-floats-value'>
+Containing or Excluding Floats</h4>
+
+	<em>This section is non-normative.</em>
+
+	Although <a>block box</a> boundaries are typically pervious to floats,
+	sometimes an author needs them to contain their own (descendant) floats
+	or to exclude floats from outside.
+	For Block layout,
+	specifying ''display: flow-root''
+	will make the box a <a>formatting context</a> root,
+	which has this behavior.
+
+	Note: Boxes participating in Flex, Grid, or Table layout will automatically have this behavior.
 
 <h3 id='column-sizing'>
 Column Sizing Keywords</h3>