Improve change-markers and diffs in some of j.u.concurrent.

- Added some missing markers.
- Added more information to some markers.
- Reverted a diff in CountedCompleter javadoc. A reference to
  java.nio.channels.CompletionHandler had been removed, presumably
  because it didn't exist at API 24 when this class was added, but it
  was added at API 26, so the reference can be restored.
- Reverted a diff in ThreadLocalRandom. A blank line had been removed,
  I'm not sure why.

Bug: 74233057
Test: make
Change-Id: Ief9315bb878bad650e42827df97ba75d47a66cef
11 files changed