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

[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).

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Commit messages:
 - Improved java.lang.ref.Cleaner javadocs.

Changes: https://git.openjdk.java.net/jdk/pull/6076/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6076&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276700
  Stats: 3 lines in 1 file changed: 2 ins; 0 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