[css-overflow-3] Add note that masking isn't affected; describe open issue for clip/clip-path.

Author: fantasai

css-overflow-3/Overview.bs

@@ -163,9 +163,11 @@ Scrollable overflow</h3>
 		    provided they themselves have ''overflow: visible''
 		    (i.e. do not themselves trap the overflow)
 		    and that overflow is not already clipped
-		    by the 'clip' or 'clip-path' properties.
+		    by the 'clip' property.
 
-		    Issue: What about masking?
+			Note: The 'masking' properties do not affect the <a>scrollable overflow region</a>.
+
+			Issue: Need to evaluate compat of honoring or ignoring 'clip' and 'clip-path'.
 	</ul>
 
 	<p class="issue">