[css-contain] Clarify phrase

This is an editorial adjustment, drawing from the non controversial
parts of #4931. This does not on its own resolve that issue.

Author: frivoal

css-contain-1/Overview.bs

@@ -467,7 +467,8 @@ Size Containment</h3>
 		</wpt>
 
 		Then,
-		its contents must then be laid out into the [=size-containment/containing box=]'s resolved size.
+		its contents must then be laid out into the [=size-containment/containing box=]'s resolved size normally (including any pseudo-elements),
+		i.e. [=size containment=] has no effect in this phase.
 
 		Note: [=size containment=] does not suppress baseline alignment.
 		See [=layout containment=] for that.

css-contain-2/Overview.bs

@@ -516,7 +516,8 @@ Size Containment</h3>
 		</wpt>
 
 		Then,
-		its contents must then be laid out into the [=size containment/containing box=]'s resolved size.
+		its contents must then be laid out into the [=size-containment/containing box=]'s resolved size normally (including any pseudo-elements),
+		i.e. [=size containment=] has no effect in this phase.
 
 		Note: [=size containment=] does not suppress baseline alignment.
 		See [=layout containment=] for that.