RFR: 8319378: Spec for j.util.Timer::purge and j.util.Timer::cancel could be improved

[Justin Lu]

Please review this PR which clarifies the definition of a _cancelled_ task in _j.util.Timer::purge_ and _j.util.Timer::cancel_.

Timer::purge claims that its return value is the number of tasks in the queue that were cancelled. This can be misleading, as a user can call Timer::cancel, thinking the rest of the tasks in the queue will be canceled (which should be the return value of Timer::purge).

In actuality, Timer::cancel _discards_ all of the tasks in the queue. For a task to have been _cancelled_, the task itself must have called TimerTask::cancel. 

This change emphasizes the difference between _discarding_ and _cancelling_ a task.
Additionally, this change also includes a drive-by update to use an _apiNote_ and _implNote_ tag in the class description.

-------------

Commit messages:
 - init

Changes: https://git.openjdk.org/jdk/pull/16503/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16503&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319378
  Stats: 21 lines in 1 file changed: 7 ins; 2 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/16503.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16503/head:pull/16503

PR: https://git.openjdk.org/jdk/pull/16503