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 <a8e3be725ec87e0d90dbf18d1fde7b2520532cc7@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/245
Tested-by: jenkins-iotivity <09cb29e8a2b473a2c978382eec13ee06fa017bda@opendaylight.org>
Reviewed-by: Uze Choi <3a26c91d8a4a3c9e246e43339f4782bb2483aece@samsung.com>
(cherry picked from commit a3c86a7a2d2fce37f68fd3cf738500e17480cbbe)
Reviewed-on: https://gerrit.iotivity.org/gerrit/378
Reviewed-by: Sudarshan Prasad <18992b2df49d20c769d3f89a8cc0a9f38a9dccaa@intel.com>

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