Update dartdoc for gpu.dart (#54529)

Update dartdoc for package:flutter_gpu with:
1) details on how to use the sdk dep
2) the current docs link

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: mit-mit

lib/gpu/lib/gpu.dart

@@ -2,13 +2,22 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-/// The Flutter GPU library.
+/// Flutter GPU is a low level API for building rendering packages from scratch.
 ///
-/// To use, import `package:flutter_gpu/gpu.dart`.
+/// To use, first add an SDK dependency in your `pubspec.yaml` file:
+/// ```
+/// dependencies:
+///   flutter_gpu:
+///     sdk: flutter
+/// ```
+/// And then add an import statement in your Dart files:
+/// ```dart
+/// import `package:flutter_gpu/gpu.dart`;
+/// ```
 ///
 /// See also:
 ///
-///  * [Flutter GPU Wiki page](https://github.com/flutter/flutter/wiki/Flutter-GPU).
+///  * [Flutter GPU documentation](https://github.com/flutter/engine/blob/main/docs/impeller/Flutter-GPU.md).
 library flutter_gpu;
 
 import 'dart:ffi';