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 <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/comedi/comedidev.h

···
  * handler will be called with the COMEDI device structure's board_ptr member
  * pointing to the matched pointer to a board name within the driver's private
  * array of static, read-only board type information.
+ *
+ * The @detach handler has two roles.  If a COMEDI device was successfully
+ * configured by the @attach or @auto_attach handler, it is called when the
+ * device is being deconfigured (by the %COMEDI_DEVCONFIG ioctl, or due to
+ * unloading of the driver, or due to device removal).  It is also called when
+ * the @attach or @auto_attach handler returns an error.  Therefore, the
+ * @attach or @auto_attach handlers can defer clean-up on error until the
+ * @detach handler is called.  If the @attach or @auto_attach handlers free
+ * any resources themselves, they must prevent the @detach handler from
+ * freeing the same resources.  The @detach handler must not assume that all
+ * resources requested by the @attach or @auto_attach handler were
+ * successfully allocated.
  */
 struct comedi_driver {
 	/* private: */