Ready: Reorg of bucket types doc

This middle one sounds like a convergence of #1 and #3. It's also worth nothing that bucket types manage a collection of buckets, allowing you to make sweeping changes across thousands of buckets in one go.
Bucket types also allow you to create thousands of similarly custom buckets without overhead. Previously, any divergence from the default bucket type forced Riak to gossip every bucket. This limited you on how many you could reasonably support.