Clarify Three-letter time zone IDs in java.util.TimeZone

There currently exists no full list of the deprecated Three-letter Java time zone IDs in TimeZone. As there is no API that also distinguishes this either, the specification should be updated to at least provide a full list (or link to ZoneId's full list). In addition, it should be clarified that there also exists three-letter IDs that are NOT deprecated, and come from the tzdb.