RFR: 8280832: Update usage docs for NonblockingQueue

[Kim Barrett]

Please review this update of the usage and implementation comments for
NonblockingQueue to discuss the ABA issue in push/append operations.

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Commit messages:
 - document ABA for push/append

Changes: https://git.openjdk.java.net/jdk/pull/7393/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7393&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280832
  Stats: 7 lines in 2 files changed: 5 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7393.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7393/head:pull/7393

PR: https://git.openjdk.java.net/jdk/pull/7393