Expose or document more options for MLoad and Export

[plorenzatto]

Would it be possible to expose or document the add_attribute method of MLoad and Export objects?

For context I'm working with a Teradata instance that severely limits the number of active sessions. After some research I found this was possible by using add_attribute method of the mload object belonging to the BulkLoad operation and then looking for the appropiate constants:

with giraffez.BulkLoad(table=table_name, **giraffez_opts) as load:
   load.mload.add_attribute(giraffez.constants.TD_MIN_SESSIONS, 1)
   load.mload.add_attribute(giraffez.constants.TD_MAX_SESSIONS, 2)
   [...]