RFR: 8286983: Add mention of "Preview Feature" for new loom related jdb and debug agent options [v2]

[Chris Plummer]

On Thu, 26 May 2022 02:35:08 GMT, David Holmes <dholmes at openjdk.org> wrote:

>>> If virtual threads become permanent then the usage message would minimally be updated to drop the sentence that virtual threads are a preview feature.
>> 
>> But that's precisely the commitment to this flag that I want to avoid.  I want something that is the equivalent of `EXPERIMENTAL` for `-XX` flags.
>
> Again I have to say I agree with Chris. The flag itself is a "preview" flag and relates to a "preview feature". If the feature goes obviously the flag does too, but the flag could go even if the feature stays.

Please see the discussion with Alex in [JDK-8286983](https://bugs.openjdk.java.net/browse/JDK-8286983)

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PR: https://git.openjdk.java.net/jdk/pull/8780