Improve hitTestSelf and hitTestChildren docs (#4484)

Fixes #4427

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

@@ -1177,6 +1177,9 @@ abstract class RenderBox extends RenderObject {
 
   /// Override this function if this render object can be hit even if its
   /// children were not hit.
+  ///
+  /// Used by [hitTest]. If you override [hitTest] and do not call this
+  /// function, then you don't need to implement this function.
   @protected
   bool hitTestSelf(Point position) => false;
 
@@ -1186,6 +1189,9 @@ abstract class RenderBox extends RenderObject {
   /// Typically children should be hit tested in reverse paint order so that
   /// hit tests at locations where children overlap hit the child that is
   /// visually "on top" (i.e., paints later).
+  ///
+  /// Used by [hitTest]. If you override [hitTest] and do not call this
+  /// function, then you don't need to implement this function.
   @protected
   bool hitTestChildren(HitTestResult result, { Point position }) => false;