Add API explained comments for RCSDiscoveryManager and RCSDiscoveryManagerImpl

Add API explained comments for RCSDiscoveryManager and RCSDiscoveryManagerImpl

- Modify return type (void -> unique_ptr<DiscoveryTask>) in RCSDiscoveryManager
- Add new API in RCSDiscoveryManagerImpl

Need to be cherry picked master branch into 1.0.0-dev branch

Change-Id: I69c90a7448efa325941c2aef53ea4a252e2ab6c8
Signed-off-by: ChaJiwon <jw_wonny.cha@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/3021
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Hun-je Yeon <hunje.yeon@samsung.com>
Reviewed-by: Habib Virji <habib.virji@samsung.com>