RFR: JDK-8312058: Documentation improvements for subscription based listeners

[John Hendrikx]

Incorporates review comments from #1069

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Commit messages:
 - Documentation improvements

Changes: https://git.openjdk.org/jfx/pull/1177/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1177&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312058
  Stats: 18 lines in 3 files changed: 7 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jfx/pull/1177.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1177/head:pull/1177

PR: https://git.openjdk.org/jfx/pull/1177