Export FlutterSemanticsUpdateNotification and improve docs (#8203)

* Export FlutterSemanticsUpdateNotification and improve docs

shell/platform/darwin/ios/framework/Headers/FlutterEngine.h

@@ -146,10 +146,16 @@ FLUTTER_EXPORT
  * This method must only be called after launching the engine via
  * `-runWithEntrypoint:` or `-runWithEntryPoint:libraryURI`.
  *
+ * Although this method returns synchronously, it does not guarantee that a
+ * semantics tree is actually available when the method returns. It
+ * synchronously ensures that the next frame the Flutter framework creates will
+ * have a semantics tree.
+ *
  * You can subscribe to semantics updates via `NSNotificationCenter` by adding
  * an observer for the name `FlutterSemanticsUpdateNotification`.  The `object`
  * parameter will be the `FlutterViewController` associated with the semantics
- * update.
+ * update.  This will asynchronously fire after a semantics tree has actually
+ * built (which may be some time after the frame has been rendered).
  */
 - (void)ensureSemanticsEnabled;
 

shell/platform/darwin/ios/framework/Headers/FlutterViewController.h

@@ -23,6 +23,7 @@
  * The object passed as the sender is the `FlutterViewController` associated
  * with the update.
  */
+FLUTTER_EXPORT
 extern NSNotificationName const FlutterSemanticsUpdateNotification;
 
 /**