Added a description for the OCQualityOfService enum

Added a description for the OCQualityOfService enum

Previously, there was no description of the meaning of the fields.  The
description added in this commit is based on dev mailing list discussion
of the field.

Change-Id: Iba183aeab69905c226c54935c15da7aeb5d522d5
Signed-off-by: William R. Dieter <william.r.dieter@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/867
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Vijay Kesavan <vijay.s.kesavan@intel.com>
Reviewed-by: Patrick Lankswert <patrick.lankswert@intel.com>