RFR: 8314480: Memory ordering spec updates in java.lang.ref

[Brent Christian]

Classes in the `java.lang.ref` package would benefit from an update to bring the spec in line with how the VM already behaves. The changes would focus on _happens-before_ edges at some key points during reference processing.

A couple key things we want to be able to say are:
- `Reference.reachabilityFence(x)` _happens-before_ reference processing occurs for 'x'.
- `Cleaner.register()` _happens-before_ the Cleaner thread runs the registered cleaning action.

This will bring Cleaner in line (or close) with the memory visibility guarantees made for finalizers in [JLS 17.4.5](https://docs.oracle.com/javase/specs/jls/se18/html/jls-17.html#jls-17.4.5):
_"There is a happens-before edge from the end of a constructor of an object to the start of a finalizer (§12.6) for that object."_

-------------

Commit messages:
 - Merge
 - Update enqueue() docs, add warnings about still-reachable referent
 - small tweak to enqueue()
 - Updates to clear() and enqueue()
 - updates re: reference clearing
 - Per slack, 'on a queue' is inconsistent with existing wording - reworked
 - Update 'remains reachable', per Slack
 - tweak again
 - tweak
 - consistency tweaks
 - ... and 28 more: https://git.openjdk.org/jdk/compare/cc4b0d92...dfce0dd5

Changes: https://git.openjdk.org/jdk/pull/16644/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16644&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314480
  Stats: 108 lines in 4 files changed: 87 ins; 0 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/16644.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16644/head:pull/16644

PR: https://git.openjdk.org/jdk/pull/16644