RFR: JDK-8312058: Documentation improvements for subscription based listeners [v3]

[John Hendrikx]

> Incorporates documentation review comments from #1069.
> 
> This PR should be kept minimal so it can be backported to JFX21; it should only contain documentation changes.

John Hendrikx has updated the pull request incrementally with one additional commit since the last revision:

  Improve Subscription javadoc

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1177/files
  - new: https://git.openjdk.org/jfx/pull/1177/files/7382af03..b943340f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1177&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1177&range=01-02

  Stats: 11 lines in 1 file changed: 9 ins; 0 del; 2 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