Document the EIO error since this can happen anywhere anytime and applications
should be aware of this.

Also fix typo: exaustive -> exhaustive
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>