PYCBC-235: Document MultiResult class more extensively

Change-Id: I3e70aa0de3790e0a2996f11fbb38ed0c45b09693
Reviewed-on: http://review.couchbase.org/36566
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>