Add API description of Things Manager.

Add API description of Things Manager.

joinGroup and findGroup function call were ambiguous
when join to a local(or remote) resource.
We provide more detail description of function for the user.

Fix [IOT-265],[IOT-267]

Change-Id: I43de1f6671dc73d0725bdc9c0e85ffca45878d2e
Signed-off-by: HyunJun Kim <hyunjun2.kim@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/245
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Uze Choi <uzchoi@samsung.com>
(cherry picked from commit a3c86a7a2d2fce37f68fd3cf738500e17480cbbe)
Reviewed-on: https://gerrit.iotivity.org/gerrit/378
Reviewed-by: Sudarshan Prasad <sudarshan.prasad@intel.com>