[docs] Document 'cas' property in OperationResult
[docs] Document 'cas' property in OperationResult

Change-Id: I8cb80309393c02fb7fe3019c694f6401986670d3
Reviewed-on: http://review.couchbase.org/27401
Tested-by: Mordechai Nunberg <e7d42768707bf23038325b0b68d4b577e5f6064a@haskalah.org>
Reviewed-by: Volker Mische <fb414f8ac0dbbf87663550ae4ef5fc95b1041941@gmail.com>

couchbase/couchbase-python-client,mnunberg/couchbase-python-client,mnunberg/couchbase-python-client,couchbase/couchbase-python-client