RFR: 8351067: doc: Clarify Platform::accessibilityActive threading use

[Andy Goryachev]

Changed the spec to require access only from the FX application thread.

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Commit messages:
 - javadoc

Changes: https://git.openjdk.org/jfx/pull/1728/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1728&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351067
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jfx/pull/1728.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1728/head:pull/1728

PR: https://git.openjdk.org/jfx/pull/1728