[css-overflow-3] Note about overflow:hidden when the ICB isn't the viewport

See #5646

Author: frivoal

css-overflow-3/Overview.bs

@@ -658,6 +658,11 @@ Overflow Viewport Propagation</h3>
 	this special handling of the HTML <{body}> element.
 	See the [[CSS-CONTAIN-1#contain-property]] for details.
 
+	Note: ''overflow: hidden'' on the root element
+	might not clip everything outside the [=Initial Containing Block=]
+	if the ICB is smaller than the viewport,
+	which can happen on mobile.
+
 	If ''overflow/visible'' is applied to the viewport,
 	it must be interpreted as ''overflow/auto''.
 	If ''overflow/clip'' is applied to the viewport,