docs(android_alarm_manager_plus): Fix example and docs to explain required annotation

[vbuberen]

Description

Fix for a problem with android_alarm_manager_plus not working in release mode on Flutter 3.3.0 and newer.
It seems that due to recent changes to a tree-shaking quite a few plugins started to have issues with isolates when app is built in release mode.

The main requirement is to annotate the callback function with @pragma('vm:entry-point') in the app where the plugin is used. So I have updated the README and docs to mention this requirement.

Here is the list with similar issues or issues on topic:
flutter/flutter#109248
firebase/flutterfire#9446
https://www.reddit.com/r/flutterhelp/comments/x3tq8l/package_not_found_in_release_but_there_in_debug/

And this is the issue about missing documentation update about the change that broke multiple plugins in the community:
flutter/website#7564

Related Issues

Closes #1077

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the pubspec.yaml files.
• All existing and new tests are passing.
• The analyzer (flutter analyze) does not report any problems on my PR.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate that with a ! in the title as explained in Conventional Commits).
• No, this is not a breaking change.

[miquelbeltran]

The pragma vm entry solution was already mentioned before: #1061 but I wasn't sure if it worked or not.

Thanks for adding this to the docs!

[vbuberen]

but I wasn't sure if it worked or not.

I have tested on our example app and it worked after adding this annotation.

[github-actions]

Visit the preview URL for this PR (updated for commit c46f7a3):

https://flutter-community-plus--pr1229-update-alarm-manager-ha460d6b.web.app

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