manager: fix Manager::createSinkClient()

Manager::createSinkClient() returns a nullptr set shared_ptr
when called with an id already associated to a sink client.
This is not the waiting behaviour: returns the existing client.

This patch fixes that and also add documentation to the public
API to explicit this behaviour.

Change-Id: Idc9a0b4f7d7a06e2bdafe965891469903f0d5d99