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

[Roger Riggs]

On Fri, 22 Oct 2021 08:03:34 GMT, Hendrik Schreiber <hschreiber 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.

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