RFR: 8269417: Minor clarification on NonblockingQueue utility

[Man Cao]

Hi,

Could you review this change mainly based on the comments in https://github.com/openjdk/jdk/pull/4379?
I also added an assertion to ensure and better understand why the Atomic::store() in append() is correct.

Stress tested with fastdebug build with:
$ make test TEST="gtest:NonblockingQueueTestBasics gtest:NonblockingQueueTest" GTEST="REPEAT=-1"

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

Commit messages:
 - Clarify NonblockingQueue's comments and add an assertion.

Changes: https://git.openjdk.java.net/jdk/pull/4600/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4600&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269417
  Stats: 25 lines in 2 files changed: 15 ins; 5 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4600.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4600/head:pull/4600

PR: https://git.openjdk.java.net/jdk/pull/4600