Added info about job cancellation (#3518)
Added info about job cancellation (#3518)

* Added info about job cancellation

* Addressed comments

* Update docs/design/jobcancellation.md

Co-authored-by: Alexander Smolyakov <3c63400f7f514ee43156049d9db0b17588f6c667@microsoft.com>

Co-authored-by: Alexander Smolyakov <3c63400f7f514ee43156049d9db0b17588f6c667@microsoft.com>
Microsoft/vsts-agent,Microsoft/vsts-agent,Microsoft/vsts-agent,Microsoft/vsts-agent