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.
Minor edits to correct typos and address review feedback
Adjust wording of pb_backlog tuning section and fix typo from review feedback
Update to Riak CS configuration doc based on review comments
polish language a bit
polish up configuring riak doc
Merge pull request #1218 from basho/feature/klm/cs-connection-pools

Document the Riak CS connection pool settings