RFR: JDK-8312058: Documentation improvements for subscription based listeners [v2]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Use @implSpec tag
 - Merge branch 'master' of https://git.openjdk.org/jfx into feature/subscriptions-doc-improvements
 - Documentation improvements

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1177/files
  - new: https://git.openjdk.org/jfx/pull/1177/files/73fb1c64..7382af03

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1177&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1177&range=00-01

  Stats: 381838 lines in 5915 files changed: 214648 ins; 124341 del; 42849 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