staging: comedi: comedidev.h: Document usage of 'detach' handler

Document when the "detach" handler function pointed to by the `detach`
member of a `struct comedi_driver` is called by the comedi core, and how
the "attach" and "auto_attach" handlers can defer clean-up to it when
they return an error to the comedi core.  This is already mentioned as
part of the documentation for `comedi_auto_config()`, but is useful to
document it for `struct comedi_driver` as well, since
`comedi_auto_config()` is not usually called directly by low-level
comedi drivers, and it is not called at all for "legacy" comedi devices
that are configured manually.

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