mtd: ABI documentation: clarification of bitflip_threshold

The -EUCLEAN return value applies to mtd_read_oob() as well as mtd_read(), but
only mtd_read() was mentioned in the blurd on bitflip_threshold in the ABI
documentation.

Signed-off-by: Mike Dunn <mikedunn@newsguy.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>