staging: comedi: comedi.h: add kernel-doc to common enums

Add "kernel-doc"-formatted comments to COMEDI's general-purpose `enum`
type declarations.  Don't bother documenting the low-level
driver-specific ones for now.

Move the declaration of `enum comedi_counter_status_flags` next to the
other general-purpose `enum` types.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>