Dialog: Added note explaining that _on() is safe to use for shared handlers because of stacking.

ui/jquery.ui.dialog.js

@@ -757,6 +757,8 @@ $.widget( "ui.dialog", {
 		if ( !this.document.data( "ui-dialog-overlays" ) ) {
 
 			// Prevent use of anchors and inputs
+			// Using _on() for an event handler shared across many instances is
+			// safe because the dialogs stack and must be closed in reverse order
 			this._on( this.document, {
 				focusin: function( event ) {
 					if ( isOpening ) {