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.
Anywhere that `app.config` is used, this should be labeled `appconfig`. I see now that this is a more general issue in the CS docs, so feel free and leave it as is (including the example I pointed out above). I'll add this to the list of more general changes that need to be made.