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.
Pushed a commit to make the suggested changes. You mentioned wordy. Do yo think the detailed explanation for why the setting change is needed is overkill here or is it useful to leave in?