In http://christophe.varoqui.free.fr/refbook.html, "the multipathd daemon" pargagraph states:
"This daemon executes the external multipath config tool when events occur."
As far as I understand from the current code this is no more true, for multipathd uses libmultipath to create new maps, and not real execution of an external binary.
(And.. of course it could be easier if docs were moved to some wiki space..)
Host Side Development Team