RFR: 8234131: Miscellaneous changes imported from jsr166 CVS 2020-12 [v3]

[Petr Janeček]

On Wed, 9 Dec 2020 18:41:02 GMT, Pavel Rappo <prappo at openjdk.org> wrote:

>> Martin Buchholz has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains one commit:
>> 
>>   JDK-8234131
>
> The changes to doc comments look good.
> 
> Thanks for incorporating my earlier code snippet suggestions published on [concurrency-interest](http://cs.oswego.edu/pipermail/concurrency-interest/2020-November/017264.html)!

This also fixes issue [JDK-8257671](https://bugs.openjdk.java.net/browse/JDK-8257671): _ThreadPoolExecutor.Discard*Policy: rejected tasks are not cancelled_, by updating the `TPE.Discard*Policy` JavaDocs.

I filed it days before the code went in as I did not see the JavaDoc update in JDK, sorry for my impatience and thank you for the fix. I cannot update the Jira, please link it to this PR, too.

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PR: https://git.openjdk.java.net/jdk/pull/1647