RFR: 8276700: Improve java.lang.ref.Cleaner javadocs [v3]

[Hendrik Schreiber]

> 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).

Hendrik Schreiber has updated the pull request incrementally with one additional commit since the last revision:

  Removed duplicate info from comment.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6076/files
  - new: https://git.openjdk.java.net/jdk/pull/6076/files/3a552acc..f5d4ee92

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6076&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6076&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6076.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6076/head:pull/6076

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