Staging: iio: lots of doc fixes

Fix iio header files kernel-doc notation errors, spelling, typos,
indentation, grammar, etc.

It would also be good if these function names were spelled
correctly, but I didn't change them:
  iio_push_or_escallate_ring_event()
  iio_trigger_dettach_poll_func()

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>