fix: update docs for task::notification::Notifier

[j-g00da]

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We appreciate the time and effort you've put into this pull request.
To help us review it efficiently, please ensure you've gone through the following checklist:

Submission Checklist 📝

• I have updated existing examples or added new ones (if applicable).
• I have used cargo fmt command to ensure that all changed code is formatted correctly.
• I have used cargo clippy command to ensure that all changed code passes latest Clippy nightly lints.
• My changes were added to the CHANGELOG.md in the proper section.

Pull Request Details 📖

Description

The docs for task::notification::Notifier methods were outdated:

• referenced Monitor struct, that was renamed to Notification,
• referenced a Drop trait, that is not implemented anymore,
• described implementation details that are no longer accurate.

Testing

N/A

[ivmarkov]

Thanks!