where valid methods with corresponding keys are:
.TP
.B CALLOUT
-calling external program, that returns a string to match
+calling external program, that returns a string to match. The
+.BR BUS
+key is optional, but if specified, the sysfs device bus must be able to be
+detemined by a "device" symlink.
.br
.RB "keys: " BUS ", " PROGRAM ", " ID
.TP