Add a remark about canceling send done with the scheduler

[mlinnosk]

Description

It can be misleading for the user that when an actor using the scheduler is stopped or restarted the send tasks are not automatically cancelled. This can lead to resource leaks or duplicate sends when the actor is restarted.

Purpose

This pull request is a:

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[CLAassistant]

All committers have signed the CLA.