RFR: 8351067: doc: Clarify Platform::accessibilityActive threading use [v2]

[Andy Goryachev]

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

Andy Goryachev has updated the pull request incrementally with one additional commit since the last revision:

  review comments

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1728/files
  - new: https://git.openjdk.org/jfx/pull/1728/files/4aa52a54..6a1a152e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1728&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1728&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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