WebSockets Next: clarify connector API docs/javadoc

[mkouba]

• also add tests for programmatic lookup of connectors
• related to Quarkus Websockets Next client connector is not reusable #44465

[quarkus-bot]

Status for workflow Quarkus Documentation CI

This is the status report for running Quarkus Documentation CI on commit c59a6dc.

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It should be safe to merge provided you have a look at the other checks in the summary.

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[github-actions]

🙈 The PR is closed and the preview is expired.

[quarkus-bot]

Status for workflow Quarkus CI

This is the status report for running Quarkus CI on commit c59a6dc.

✅ The latest workflow run for the pull request has completed successfully.

It should be safe to merge provided you have a look at the other checks in the summary.

You can consult the Develocity build scans.

[gsmet]

If you want this in 3.17, please add the backport label as 3.17 was branched this morning, thanks!