Document how to size IV's child correctly, after seeing confusion in Github issues (#64100)

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

@@ -19,8 +19,23 @@ import 'ticker_provider.dart';
 ///
 /// The user can transform the child by dragging to pan or pinching to zoom.
 ///
+/// By default, InteractiveViewer may draw outside of its original area of the
+/// screen, such as when a child is zoomed in and increases in size. However, it
+/// will not receive gestures outside of its original area. To prevent
+/// InteractiveViewer from drawing outside of its original size, wrap it in a
+/// [ClipRect]. Or, to prevent dead areas where InteractiveViewer does not
+/// receive gestures, be sure that the InteractiveViewer widget is the size of
+/// the area that should be interactive. See
+/// [flutter-go](https://github.com/justinmc/flutter-go) for an example of
+/// robust positioning of an InteractiveViewer child that works for all screen
+/// sizes and child sizes.
+///
 /// The [child] must not be null.
 ///
+/// See also:
+///   * The [Flutter Gallery's transformations demo](https://github.com/flutter/gallery/blob/master/lib/demos/reference/transformations_demo.dart),
+///     which includes the use of InteractiveViewer.
+///
 /// {@tool dartpad --template=stateless_widget_scaffold}
 /// This example shows a simple Container that can be panned and zoomed.
 ///