RFR: JDK-8312058: Documentation improvements for subscription based listeners [v6]

[Nir Lisker]

On Fri, 11 Aug 2023 22:03:28 GMT, John Hendrikx <jhendrikx at openjdk.org> wrote:

>> 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:
> 
>   Update wording on value subscriber

The changes to `Observable/Value` seem significant enough to not be part of this change. Approving the current changes.

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

Marked as reviewed by nlisker (Reviewer).

PR Review: https://git.openjdk.org/jfx/pull/1177#pullrequestreview-1586660488