staging: comedi: drivers.c: document exported functions

Add missing kernel-doc to the low-level COMEDI driver API functions
exported from "drivers.c" and tart up some of the existing kernel-doc
comments for consistency.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>