link "iOS PlatformView BackdropFilter design doc" in the BackdropFilter widget's documentation (#113779)

Author: Chris Yang

packages/flutter/lib/src/widgets/basic.dart

@@ -539,6 +539,7 @@ class ShaderMask extends SingleChildRenderObjectWidget {
 ///  * [ImageFiltered], which applies an [ImageFilter] to its child.
 ///  * [DecoratedBox], which draws a background under (or over) a widget.
 ///  * [Opacity], which changes the opacity of the widget itself.
+///  * https://flutter.dev/go/ios-platformview-backdrop-filter-blur for details and restrictions when an iOS PlatformView needs to be blurred.
 class BackdropFilter extends SingleChildRenderObjectWidget {
   /// Creates a backdrop filter.
   ///