Add brief descriptions to several classes in things manager feature

Add brief descriptions to several classes in things manager feature

GroupManager, ThingsConfiguration, and ThingsMaintenance has been added with
brief descriptions.

Change-Id: Ic2f884affa1754ff2b897eda171715624641b58b
Signed-off-by: Jihun Ha <jihun.ha@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/2969
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Madan Lanka <lanka.madan@samsung.com>
(cherry picked from commit 82a52fd3586e09fd5828c23b54a2e0bfdbe77a61)
Reviewed-on: https://gerrit.iotivity.org/gerrit/2977