Added warning message for --sleep

[Glennmen]

I had --sleep=60 in my Supervisor config and this caused an issue that stopped my worker after 1 minute. I didn't know about --timeout and that it doesn't see a "sleeping" process as an active worker.
I hope by adding this little warning I can save other people the time of debugging this issue.
Also made a blog post about the issue: DEV.to

I also am not sure what the target branch should be, because older versions could also benefit from this warning message. I was working on a Laravel 5.7 project when I had this issue.

[taylorotwell]

Honestly, I'm not sure it would be common to ever set a sleep so high.

[Glennmen]

@taylorotwell In my case I set it this high because there won't be that much jobs and I thought it was unnecessary to open connections to Redis.
But it is your call if the warning is useful or not, I know that in my case it would have saved me the debugging time.

[realtebo]

I upvote this pull request just to clarify this behaviour on documentation.