Document the Riak CS connection pool settings

Add documentation about the Riak CS connection pool settings. Describe
the difference between the two connection pools currently used by Riak
CS, describe the meaning of the configuration pair, and provide some
guidance on sizing and tuning.
Good link juice opportunity (relevant text in the link). Also, passive voice. Also, wordy :)

"We strongly recommend you [[increase the value of the `pb_backlog` setting|Configuring Riak for CS#Setting-Up-Riak-to-Use-Protocol-Buffers]] in Riak."