ticket:8464: Improve documentation on setNumberFormat.

git-svn-id: http://source.icu-project.org/repos/icu/icu4j/branches/tkeep@34583 251d0590-4201-4cf1-90de-194747b24ca1
ticket:8464: RelativeFormatter -> RelativeDateTimeFormatter.

git-svn-id: http://source.icu-project.org/repos/icu/icu4j/branches/tkeep@34584 251d0590-4201-4cf1-90de-194747b24ca1