firewire: Add more documentation to firewire-cdev.h

 * @request:       Valid if @common.type == %FW_CDEV_EVENT_REQUEST

 * @iso_interrupt: Valid if @common.type == %FW_CDEV_EVENT_ISO_INTERRUPT

 *

 * Convenience union for userspace use.  Events could be read(2) into a char

 * buffer and then cast to this union for further processing.

 * Convenience union for userspace use.  Events could be read(2) into an

 * appropriately aligned char buffer and then cast to this union for further

 * processing.  Note that for a request, response or iso_interrupt event,

 * the data[] or header[] may make the size of the full event larger than

 * sizeof(union fw_cdev_event).  Also note that if you attempt to read(2)

 * an event into a buffer that is not large enough for it, the data that does

 * not fit will be discarded so that the next read(2) will return a new event.

 */

union fw_cdev_event {

	struct fw_cdev_event_common common;