Minor documentation fixes (#70501)

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

@@ -53,6 +53,11 @@ abstract class RenderAbstractViewport extends RenderObject {
   /// Returns the offset that would be needed to reveal the `target`
   /// [RenderObject].
   ///
+  /// This is used by [RenderViewportBase.showInViewport], which is
+  /// itself used by [RenderObject.showOnScreen] for
+  /// [RenderViewportBase], which is in turn used by the semantics
+  /// system to implement scrolling for accessibility tools.
+  ///
   /// The optional `rect` parameter describes which area of that `target` object
   /// should be revealed in the viewport. If `rect` is null, the entire
   /// `target` [RenderObject] (as defined by its [RenderObject.paintBounds])

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

@@ -3326,6 +3326,15 @@ abstract class Element extends DiagnosticableTree implements BuildContext {
   /// | :-----------------: | :--------------------- | :---------------------- |
   /// |  **child == null**  |  Returns null.         |  Returns new [Element]. |
   /// |  **child != null**  |  Old child is removed, returns null. | Old child updated if possible, returns child or new [Element]. |
+  ///
+  /// The `newSlot` argument is used only if `newWidget` is not null. If `child`
+  /// is null (or if the old child cannot be updated), then the `newSlot` is
+  /// given to the new [Element] that is created for the child, via
+  /// [inflateWidget]. If `child` is not null (and the old child _can_ be
+  /// updated), then the `newSlot` is given to [updateSlotForChild] to update
+  /// its slot, in case it has moved around since it was last built.
+  ///
+  /// See the [RenderObjectElement] documentation for more information on slots.
   @protected
   Element? updateChild(Element? child, Widget? newWidget, dynamic newSlot) {
     if (newWidget == null) {
@@ -5336,10 +5345,10 @@ class InheritedElement extends ProxyElement {
 /// class FooElement extends RenderObjectElement {
 ///
 ///   @override
-///   Foo get widget => super.widget;
+///   Foo get widget => super.widget as Foo;
 ///
 ///   @override
-///   RenderFoo get renderObject => super.renderObject;
+///   RenderFoo get renderObject => super.renderObject as RenderFoo;
 ///
 ///   // ...
 /// }
@@ -5397,13 +5406,13 @@ class InheritedElement extends ProxyElement {
 ///
 /// #### Dynamically determining the children during layout
 ///
-/// If the widgets are to be generated at layout time, then generating them when
-/// the [update] method won't work: layout of this element's render object
-/// hasn't started yet at that point. Instead, the [update] method can mark the
-/// render object as needing layout (see [RenderObject.markNeedsLayout]), and
-/// then the render object's [RenderObject.performLayout] method can call back
-/// to the element to have it generate the widgets and call [updateChild]
-/// accordingly.
+/// If the widgets are to be generated at layout time, then generating them in
+/// the [mount] and [update] methods won't work: layout of this element's render
+/// object hasn't started yet at that point. Instead, the [update] method can
+/// mark the render object as needing layout (see
+/// [RenderObject.markNeedsLayout]), and then the render object's
+/// [RenderObject.performLayout] method can call back to the element to have it
+/// generate the widgets and call [updateChild] accordingly.
 ///
 /// For a render object to call an element during layout, it must use
 /// [RenderObject.invokeLayoutCallback]. For an element to call [updateChild]