staging: comedi: comedi.h: add kernel-doc comments to struct types

Add "kernel-doc"-formatted comments to the COMEDI `struct` declarations
used with ioctls.  Don't bother documenting `struct comedi_trig` as it
is obsolete and not supported.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>