Improve transactional observer without jta warning

[Postremus]

When an application has a transaction observer, but does not depend on JTA, following warning is printed once on startup:

The observer %s makes use of %s transactional observers but no JTA capabilities were detected.

According to the spec:

If no transaction is in progress when the event is fired, they are notified at the same time as other observers.

This is already the behavior in case of:

• JTA is present, but no Transaction is running
• JTA is not present. In this case however, the warning is logged at startup.

This warning was therefore a bit misleading in my opinion, since I first thought that the observer would never be triggered.
In a discussion on zulip, we agreed on expanding the message.

Relevant chat: https://quarkusio.zulipchat.com/#narrow/stream/187030-users/topic/Use.20alternative.20observer