Improve Rebalance Recommendation documentation in README

[haugenj]

Clarify that the rebalance rec only applies to Spot instances and that it's a separate event type than an AZ rebalance

#416

[github-actions]

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[Xyaren]

This is also very confusing to me !

[LikithaVemulapalli]

Closing this issue, changes included in the latest release v1.20.0