Document recommendation to use a dedicated SSD for the WAL

It is a common deployment best practice to place the WAL on its own SSD to maximize ingest throughput in a Kudu cluster. However, we don't clearly document that recommendation in the places users would commonly look for it. 

It is mentioned somewhat in the FAQ:
https://kudu.apache.org/faq.html#how-does-kudu-store-its-data-is-the-underlying-data-storage-readable-without-going-through-kudu

But it should probably be mentioned in these places:
https://kudu.apache.org/docs/installation.html#prerequisites_and_requirements
https://kudu.apache.org/docs/configuration.html#directory_configuration
https://kudu.apache.org/docs/scaling_guide.html

Alternatively a hardware/deployment guide might be useful, but that is a larger undertaking.