Improve Stack docs (#8312)

Discuss how to change paint order.

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

@@ -1461,6 +1461,13 @@ class BlockBody extends MultiChildRenderObjectWidget {
 /// placed relative to the stack according to their top, right, bottom, and left
 /// properties.
 ///
+/// The stack paints its children in order. If you want to change the order in
+/// which the children paint, you can rebuild the stack with the children in
+/// the new order. If you reorder the children in this way, consider giving the
+/// children non-null keys. These keys will cause the framework to move the
+/// underlying objects for the children to their new locations rather than
+/// recreate them at their new location.
+///
 /// For more details about the stack layout algorithm, see [RenderStack].
 ///
 /// If you want to lay a number of children out in a particular pattern, or if