[css-contain-1] Update changes section

Author: frivoal

css-contain-1/Overview.bs

@@ -1330,6 +1330,15 @@ This appendix is <em>informative</em>.
 
 				<li>
 					Note that paint effects with non-local effects can limit certain optimization opportunities.
+
+				<li>
+					Add subsection headings for the possible optimisations
+
+				<li>
+					List cases where various types of containment do not apply
+					at the end rather than the start of of each section,
+					to start with the general behavior
+					and go into exceptions later.
 			</ul>
 	</ul>