Add more documentation for TextEditingController default constructor (#143452)

## Description

This PR adds more documentation for `TextEditingController(String text)` constructor and it adds one example.

https://github.com/flutter/flutter/pull/96245 was a first improvement to the documentation.
https://github.com/flutter/flutter/issues/79495 tried to hide the cursor when an invalid selection is set but it was reverted.
https://github.com/flutter/flutter/pull/123777 mitigated the issue of having a default invalid selection: it takes care of setting a proper selection when a text field is focused and its controller selection is not initialized.

I will try changing the initial selection in another PR, but It will probably break several existing tests.

## Related Issue

Fixes https://github.com/flutter/flutter/issues/95978

## Tests

Adds 1 test for the new example.

examples/api/lib/widgets/editable_text/text_editing_controller.1.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+
+/// Flutter code sample for [TextEditingController].
+
+void main() {
+  runApp(const TextEditingControllerExampleApp());
+}
+
+class TextEditingControllerExampleApp extends StatelessWidget {
+  const TextEditingControllerExampleApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const MaterialApp(
+      home: TextEditingControllerExample(),
+    );
+  }
+}
+
+class TextEditingControllerExample extends StatefulWidget {
+  const TextEditingControllerExample({super.key});
+
+  @override
+  State<TextEditingControllerExample> createState() => _TextEditingControllerExampleState();
+}
+
+class _TextEditingControllerExampleState extends State<TextEditingControllerExample> {
+  // Create a controller whose initial selection is empty (collapsed) and positioned
+  // before the text (offset is 0).
+  final TextEditingController _controller = TextEditingController.fromValue(
+    const TextEditingValue(
+      text: 'Flutter',
+      selection: TextSelection.collapsed(offset: 0),
+    ),
+  );
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      body: Container(
+        alignment: Alignment.center,
+        padding: const EdgeInsets.all(16.0),
+        child: TextField(
+          controller: _controller,
+          autofocus: true,
+          decoration: const InputDecoration(filled: true),
+        ),
+      ),
+    );
+  }
+}

examples/api/test/widgets/editable_text/text_editing_controller.1_test.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter_api_samples/widgets/editable_text/text_editing_controller.1.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Initial selection is collasped at offset 0', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.TextEditingControllerExampleApp(),
+    );
+
+    final EditableText editableText = tester.widget(find.byType(EditableText));
+    final TextEditingController controller = editableText.controller;
+
+    expect(controller.text, 'Flutter');
+    expect(controller.selection, const TextSelection.collapsed(offset: 0));
+  });
+}

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

@@ -178,6 +178,7 @@ class _RenderCompositionCallback extends RenderProxyBox {
 ///
 /// Remember to [dispose] of the [TextEditingController] when it is no longer
 /// needed. This will ensure we discard any resources used by the object.
+///
 /// {@tool dartpad}
 /// This example creates a [TextField] with a [TextEditingController] whose
 /// change listener forces the entered text to be lower case and keeps the
@@ -198,6 +199,24 @@ class TextEditingController extends ValueNotifier<TextEditingValue> {
   ///
   /// This constructor treats a null [text] argument as if it were the empty
   /// string.
+  ///
+  /// The initial selection is `TextSelection.collapsed(offset: -1)`.
+  /// This indicates that there is no selection at all ([TextSelection.isValid]
+  /// is false in this case). When a text field is built with a controller whose
+  /// selection is not valid, the text field will update the selection when it
+  /// is focused (the selection will be an empty selection positioned at the
+  /// end of the text).
+  //
+  /// Consider using [TextEditingController.fromValue] to initialize both the
+  /// text and the selection.
+  ///
+  /// {@tool dartpad}
+  /// This example creates a [TextField] with a [TextEditingController] whose
+  /// initial selection is empty (collapsed) and positioned at the beginning
+  /// of the text (offset is 0).
+  ///
+  /// ** See code in examples/api/lib/widgets/editable_text/text_editing_controller.1.dart **
+  /// {@end-tool}
   TextEditingController({ String? text })
     : super(text == null ? TextEditingValue.empty : TextEditingValue(text: text));