[jfx15] RFR: 8246343: Fix mistakes in FX API docs

[Nir Lisker]

Fixes https://bugs.openjdk.java.net/browse/JDK-8246343 and some additional fixes in the vicinity.

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Commit messages:
 - Initial commit of 8246343

Changes: https://git.openjdk.java.net/jfx/pull/274/files
 Webrev: https://webrevs.openjdk.java.net/jfx/274/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8246343
  Stats: 27 lines in 6 files changed: 3 ins; 2 del; 22 mod
  Patch: https://git.openjdk.java.net/jfx/pull/274.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/274/head:pull/274

PR: https://git.openjdk.java.net/jfx/pull/274