Add warnings and an example to the TextEditingController docs (#28604)

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

@@ -62,6 +62,50 @@ const int _kObscureShowLatestCharCursorTicks = 3;
 /// text field. If you build a text field with a controller that already has
 /// [text], the text field will use that text as its initial value.
 ///
+/// The [text] or [selection] properties can be set from within a listener
+/// added to this controller. If both properties need to be changed then the
+/// controller's [value] should be set instead.
+///
+/// {@tool snippet --template=stateful_widget_material}
+/// This example creates a [TextField] with a [TextEditingController] whose
+/// change listener forces the entered text to be lower case and keeps the
+/// cursor at the end of the input.
+///
+/// ```dart
+/// final _controller = TextEditingController();
+///
+/// void initState() {
+///   _controller.addListener(() {
+///     final text = _controller.text.toLowerCase();
+///     _controller.value = _controller.value.copyWith(
+///       text: text,
+///       selection: TextSelection(baseOffset: text.length, extentOffset: text.length),
+///       composing: TextRange.empty,
+///     );
+///   });
+///   super.initState();
+/// }
+///
+/// void dispose() {
+///   _controller.dispose();
+///   super.dispose();
+/// }
+///
+/// Widget build(BuildContext context) {
+///   return Scaffold(
+///     body: Container(
+///      alignment: Alignment.center,
+///       padding: const EdgeInsets.all(6),
+///       child: TextFormField(
+///         controller: _controller,
+///        decoration: InputDecoration(border: OutlineInputBorder()),
+///       ),
+///     ),
+///   );
+/// }
+/// ```
+/// {@end-tool}
+///
 /// See also:
 ///
 ///  * [TextField], which is a Material Design text field that can be controlled
@@ -89,10 +133,17 @@ class TextEditingController extends ValueNotifier<TextEditingValue> {
   /// that they need to update (it calls [notifyListeners]). For this reason,
   /// this value should only be set between frames, e.g. in response to user
   /// actions, not during the build, layout, or paint phases.
+  ///
+  /// This property can be set from a listener added to this
+  /// [TextEditingController]; however, one should not also set [selection]
+  /// in a separate statement. To change both the [text] and the [selection]
+  /// change the controller's [value].
   set text(String newText) {
-    value = value.copyWith(text: newText,
+    value = value.copyWith(
+      text: newText,
       selection: const TextSelection.collapsed(offset: -1),
-                           composing: TextRange.empty);
+      composing: TextRange.empty
+    );
   }
 
   /// The currently selected [text].
@@ -104,6 +155,11 @@ class TextEditingController extends ValueNotifier<TextEditingValue> {
   /// that they need to update (it calls [notifyListeners]). For this reason,
   /// this value should only be set between frames, e.g. in response to user
   /// actions, not during the build, layout, or paint phases.
+  ///
+  /// This property can be set from a listener added to this
+  /// [TextEditingController]; however, one should not also set [text]
+  /// in a separate statement. To change both the [text] and the [selection]
+  /// change the controller's [value].
   set selection(TextSelection newSelection) {
     if (newSelection.start > text.length || newSelection.end > text.length)
       throw FlutterError('invalid text selection: $newSelection');