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

[Nir Lisker]

On Thu, 13 Jul 2023 21:13:06 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.

modules/javafx.base/src/main/java/javafx/beans/Subscription.java line 63:

> 61:      *
> 62:      * Implementors must ensure the implementation is idempotent (a no-op
> 63:      * if called more than once).

I now think this should be in an `@implSpec` tag since it specifies the implementation conditions.

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PR Review Comment: https://git.openjdk.org/jfx/pull/1177#discussion_r1263201551