Docs: provide Guidance for SingleChildRenderObjectWidget implementations (#49993)

packages/flutter/lib/src/rendering/object.dart

@@ -2897,7 +2897,11 @@ abstract class RenderObject extends AbstractNode with DiagnosticableTreeMixin im
 
 /// Generic mixin for render objects with one child.
 ///
-/// Provides a child model for a render object subclass that has a unique child.
+/// Provides a child model for a render object subclass that has
+/// a unique child, which is accessible via the [child] getter.
+///
+/// This mixin is typically used to implement render objects created
+/// in a [SingleChildRenderObjectWidget].
 mixin RenderObjectWithChildMixin<ChildType extends RenderObject> on RenderObject {
 
   /// Checks whether the given render object has the correct [runtimeType] to be

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

@@ -1782,9 +1782,14 @@ abstract class LeafRenderObjectWidget extends RenderObjectWidget {
   LeafRenderObjectElement createElement() => LeafRenderObjectElement(this);
 }
 
-/// A superclass for RenderObjectWidgets that configure RenderObject subclasses
+/// A superclass for [RenderObjectWidget]s that configure [RenderObject] subclasses
 /// that have a single child slot. (This superclass only provides the storage
 /// for that child, it doesn't actually provide the updating logic.)
+///
+/// Typically, the render object assigned to this widget will make use of
+/// [RenderObjectWithChildMixin] to implement a single-child model. The mixin
+/// exposes a [RenderObjectWithChildMixin.child] property that allows
+/// retrieving the render object belonging to the [child] widget.
 abstract class SingleChildRenderObjectWidget extends RenderObjectWidget {
   /// Abstract const constructor. This constructor enables subclasses to provide
   /// const constructors so that they can be used in const expressions.