Update docs for ancestorWidgetOfExactType (#28675)

* doc update

packages/flutter/lib/src/material/about.dart

@@ -488,6 +488,12 @@ class _LicensePageState extends State<LicensePage> {
 }
 
 String _defaultApplicationName(BuildContext context) {
+  // This doesn't handle the case of the application's title dynamically
+  // changing. In theory, we should make Title expose the current application
+  // title using an InheritedWidget, and so forth. However, in practice, if
+  // someone really wants their application title to change dynamically, they
+  // can provide an explicit applicationName to the widgets defined in this
+  // file, instead of relying on the default.
   final Title ancestorTitle = context.ancestorWidgetOfExactType(Title);
   return ancestorTitle?.title ?? Platform.resolvedExecutable.split(Platform.pathSeparator).last;
 }

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

@@ -1942,11 +1942,15 @@ abstract class BuildContext {
   /// Returns the nearest ancestor widget of the given type, which must be the
   /// type of a concrete [Widget] subclass.
   ///
-  /// This should not be used from build methods, because the build context will
-  /// not be rebuilt if the value that would be returned by this method changes.
-  /// In general, [inheritFromWidgetOfExactType] is more useful. This method is
+  /// In general, [inheritFromWidgetOfExactType] is more useful, since inherited
+  /// widgets will trigger consumers to rebuild when they change. This method is
   /// appropriate when used in interaction event handlers (e.g. gesture
-  /// callbacks), or for performing one-off tasks.
+  /// callbacks) or for performing one-off tasks such as asserting that you have
+  /// or don't have a widget of a specific type as an ancestor. The return value
+  /// of a Widget's build method should not depend on the value returned by this
+  /// method, because the build context will not rebuild if the return value of
+  /// this method changes. This could lead to a situation where data used in the
+  /// build method changes, but the widget is not rebuilt.
   ///
   /// Calling this method is relatively expensive (O(N) in the depth of the
   /// tree). Only call this method if the distance from this widget to the