update reassemble doc (#35164)

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

@@ -2665,6 +2665,15 @@ abstract class Element extends DiagnosticableTree implements BuildContext {
   /// This function will only be called during development. In release builds,
   /// the `ext.flutter.reassemble` hook is not available, and so this code will
   /// never execute.
+  ///
+  /// Implementers should not rely on any ordering for hot reload source update,
+  /// reassemble, and build methods after a hot reload has been initiated. It is
+  /// possible that a [Timer] (e.g. an [Animation]) or a debugging session
+  /// attached to the isolate could trigger a build with reloaded code _before_
+  /// reassemble is called. Code that expects preconditions to be set by
+  /// reassemble after a hot reload must be resilient to being called out of
+  /// order, e.g. by fizzling instead of throwing. That said, once reassemble is
+  /// called, build will be called after it at least once.
   /// {@endtemplate}
   ///
   /// See also: