PIDS — Change description of ExternalIds trait

Summary: 1) Change description of ExternalIds trait to read "This read-only trait indicates the set of other IDs a person may have. The IDs listed may be from a different ID Domain than the ID Domain of the ID this trait is bound to. This is more general purpose than CorrelatedIds. The IDs listed in this trait may be set by other mechanisms than automatic correlation. For example, external ids are added to non-correlating domains using the IDMgr::add_external_ids operation. Correlating PIDS should return correlated ids. They should also return any non-correlated external ids if they are tracking non-correlated ids. Note that the client will not be able to tell the difference between correlated ids and non-correlated ids. Not all PIDS implementations will support this trait and the tracking of non-correlated ids." In the conformance section: "Tracking non-correlated external ids - If a PIDS service tracks non-correlated ids it must support the IdMgr::add_external_ids operation and the PIDS/ExternalIds trait."

resolved is 1836, close issue