PYCBC-110: Document C internals API

Change-Id: I1409ebf1378d43988b72744fedb04c713d461d52
Reviewed-on: http://review.couchbase.org/26288
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Reviewed-by: Mordechai Nunberg <mnunberg@haskalah.org>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>