Document suggested settings for PressureAwareCompactionThroughputController

After upgrading a cluster to 2.3, we noticed that compactions were taking much longer. We tracked this down to the PressureAwareCompactionThroughputController, which became default as part of our upgrade. Our default configuration settings set the lower bound: 50MB/s, upper bound: 100MB/s. These settings make sense if you have a 1gbit NIC (~120 MB/s). For a 10gbit NIC, they’re quite inadequate.

Let's document this component explicitly and mention it as a call-out in the section on upgrading. Bonus points if we can provide suggested configuration values for the common networking configurations.