RFR: 8269417: Minor clarification on NonblockingQueue utility [v2]

[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"

Man Cao has updated the pull request incrementally with one additional commit since the last revision:

  Clarify cmpxchg only fail due to try_pop.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4600/files
  - new: https://git.openjdk.java.net/jdk/pull/4600/files/b4150ec9..e7890729

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4600&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4600&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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