Add link to SwiftPM manual integration instructions (#168066)

If SwiftPM integration fails, display a link to instructions on how to
integrate manually.

New error message will look like
```
An error occurred when adding Swift Package Manager integration:
  Exception: Forced a throw

Swift Package Manager is currently an experimental feature, please file a bug at
  https://github.com/flutter/flutter/issues/new?template=01_activation.yml
Consider including a copy of the following files in your bug report:
  ios/Runner.xcodeproj/project.pbxproj
  ios/Runner.xcodeproj/xcshareddata/xcschemes/Runner.xcscheme (or the scheme for the flavor used)

To add Swift Package Manager integration manually, please use the following instructions:
https://docs.flutter.dev/packages-and-plugins/swift-package-manager/for-app-developers#add-to-a-flutter-app-manually

Alternatively, to avoid this failure, disable Flutter Swift Package Manager integration for the project
by adding the following in the project's pubspec.yaml under the "flutter" section:
  "disable-swift-package-manager: true"
Or disable Flutter Swift Package Manager integration globally with the
following command:
  "flutter config --no-enable-swift-package-manager"
```

Fixes https://github.com/flutter/flutter/issues/160214.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

packages/flutter_tools/lib/src/migrations/swift_package_manager_integration_migration.dart

@@ -173,8 +173,6 @@ class SwiftPackageManagerIntegrationMigration extends ProjectMigrator {
       await _xcodeProjectInterpreter.getInfo(_xcodeProject.hostAppRoot.path);
     } on Exception catch (e) {
       restoreFromBackup(schemeInfo);
-      // TODO(vashworth): Add link to instructions on how to manually integrate
-      // once available on website.
       throwToolExit(
         'An error occurred when adding Swift Package Manager integration:\n'
         '  $e\n\n'
@@ -184,10 +182,12 @@ class SwiftPackageManagerIntegrationMigration extends ProjectMigrator {
         '  ${_platform.name}/Runner.xcodeproj/project.pbxproj\n'
         '  ${_platform.name}/Runner.xcodeproj/xcshareddata/xcschemes/Runner.xcscheme '
         '(or the scheme for the flavor used)\n\n'
-        'To avoid this failure, disable Flutter Swift Package Manager integration for the project\n'
+        'To add Swift Package Manager integration manually, please use the following instructions:\n'
+        'https://docs.flutter.dev/to/add-swift-package-manager-manually\n\n'
+        'Alternatively, to avoid this failure, disable Flutter Swift Package Manager integration for the project\n'
         'by adding the following in the project\'s pubspec.yaml under the "flutter" section:\n'
         '  "disable-swift-package-manager: true"\n'
-        'Alternatively, disable Flutter Swift Package Manager integration globally with the\n'
+        'Or disable Flutter Swift Package Manager integration globally with the\n'
         'following command:\n'
         '  "flutter config --no-enable-swift-package-manager"\n',
       );