RFR: 8293839: Documentation memory consistency effects of runLater

[Jens Lidestrom]

Prior to this change it was not clear from the documentation if callers of Platform#runLater must perform
any synchronisation to have writes of the calling thread be visible in the JavaFX Application
Thread. It is important to document either if callers can rely on runLater to do such synchronisation
internally, or to document is users CAN NOT rely on runLater for this.

This change documents that actions in a thread prior to submitting a runnable to
Platform#runLater happen-before actions performed by the runnable in the JavaFX
Application Thread.

runLater inherits the memory consistency effects of InvokeLaterDispatcher in most cases. 
InvokeLaterDispatcher uses BlockingDeque internally. This change documents this
in the same way as it is documented by BlockingDeque.

Other implementations of runLater should have similar memory consistency effects.

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Commit messages:
 - Remove trailing
 - Document memory consistency effects of runLater

Changes: https://git.openjdk.org/jfx/pull/872/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=872&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293839
  Stats: 6 lines in 1 file changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/872.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/872/head:pull/872

PR: https://git.openjdk.org/jfx/pull/872