ticket:8464: Clean up documentation in RelativeFormatter.java to match API proposal.

git-svn-id: http://source.icu-project.org/repos/icu/icu4j/branches/tkeep@34306 251d0590-4201-4cf1-90de-194747b24ca1
ticket:8464: Merge r34207 to r34306 from trunk.

git-svn-id: http://source.icu-project.org/repos/icu/icu4j/branches/tkeep@34307 251d0590-4201-4cf1-90de-194747b24ca1