[jfx21] RFR: 8312058: Documentation improvements for subscription based listeners

[John Hendrikx]

Backport of https://bugs.openjdk.org/browse/JDK-8312058

-------------

Commit messages:
 - Backport be2c7aea20c7752f069e526d2c3f9bea11bc1634

Changes: https://git.openjdk.org/jfx/pull/1218/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1218&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312058
  Stats: 40 lines in 3 files changed: 27 ins; 0 del; 13 mod
  Patch: https://git.openjdk.org/jfx/pull/1218.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1218/head:pull/1218

PR: https://git.openjdk.org/jfx/pull/1218