Clarify documentation describing configuration for AWS ASGs with Docker autoscaler

What does this MR do?

A small improvement to the description of how you should set up your ASG to play nicely with the Docker autoscaler executor. Right now, the requirement to turn off AZRebalance is buried in the troubleshooting section at the bottom of the page, where it's easy to miss. This change would've saved me hours of debugging time.

Related issues

Decided to fix this directly rather than opening an issue for it, but happy to open one if that's preferred.

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