[css-overflow-3] Clarify effects of containment on propagation

Closes #7184

Author: frivoal

css-overflow-3/Overview.bs

@@ -672,7 +672,7 @@ Overflow Viewport Propagation</h3>
 	The element from which the value is propagated must then have
 	a used 'overflow' value of ''overflow/visible''.
 
-	Note: Using [=containment=] disables
+	Note: Using [=containment=] on the HTML <{html}> or <{body}> elements disables
 	this special handling of the HTML <{body}> element.
 	See the [[CSS-CONTAIN-1#contain-property]] for details.