PYCBC-182: Clarify conncache documentation

Change-Id: I72627fcc4ead484640cc25a1f588afa91cc58118
Reviewed-on: http://review.couchbase.org/29277
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>