RFR: 8297605: DelayQueue javadoc is confusing

[Martin Buchholz]

Inviting @DougLea and @viktorklang-ora to review.

As usual, I couldn't resist more fiddling.  
- Added missing tests for take, remove(), remove(Object).
- Improved DelayQueueTest's testing infrastructure
- added more test assertions
- linkified new javadoc definitions

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Commit messages:
 - JDK-8297605

Changes: https://git.openjdk.org/jdk/pull/12838/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12838&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297605
  Stats: 170 lines in 2 files changed: 114 ins; 9 del; 47 mod
  Patch: https://git.openjdk.org/jdk/pull/12838.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12838/head:pull/12838

PR: https://git.openjdk.org/jdk/pull/12838