[jfx20] RFR: 8293587: Fix mistakes in FX API docs

[Nir Lisker]

Fixes and cleanup in the areas in the linked issue.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jfx/pull/1025/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1025&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293587
  Stats: 50 lines in 6 files changed: 3 ins; 4 del; 43 mod
  Patch: https://git.openjdk.org/jfx/pull/1025.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/1025/head:pull/1025

PR: https://git.openjdk.org/jfx/pull/1025