RFR: 8276700: Improve java.lang.ref.Cleaner javadocs

[Hendrik Schreiber]

On Tue, 9 Nov 2021 14:39:49 GMT, Roger Riggs <rriggs at openjdk.org> wrote:

>> Trivial improvement.
>> 
>> Explicitly show how to create a `Cleaner` instance using `Cleaner.create()`.
>> Repeat (again) in the code example that the `State` `Runnable `should be implemented as static class and not reference the instance to be cleaned, to make the point even more clear to those people who never read the javadoc *prose*.
>> 
>> I have signed the OCA a while back as [hschreiber](https://openjdk.java.net/census#hschreiber).
>
> As for compilable code, this may be a good place to use Snippets.  
> [JEP 413: Code Snippets in Java API Documentation](https://openjdk.java.net/jeps/413)
> They are new but may offer some value add here.
> 
> Good standalone examples aren't always good programming practice.

@RogerRiggs @mlchung Could either of you please sponsor this? Thanks so much and hopefully a great start into 2022!

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PR: https://git.openjdk.java.net/jdk/pull/6076