Document flags argument in DML callbacks

From README:

spaces is a table that associates old space number and table with definitions:

• insert - custom insert function function(tuple, flags). If not defined, a function that inserts a tuple into a space with new_id.
• delete - custom delete function function(key, flags). If not defined, a function that deletes a tuple with a key from a space with new_id.
• update - custom update function function(key, ops, flags. If not defined, a function that executes update with operations ops in a space with new_id on a tuple with PK key.

Meaning of flags is not described anywhere.