Document `-s force` impact on AWS mode

[Innixma]

My team had a very confusing time when -m aws -s force caused the IAM role of AutomlBenchmarkRole to be deleted and recreated, since we have a custom AutomlBenchmarkRole which enables writing to multiple buckets. Could this be better documented?

[PGijsbers]

Definitely should be better documented, but we have a related open issue so I'll roll this remark into that one (#395).