docs: add password requirements to MyJDownloader description in quest…

[baobaollf]

Update password hint for JDownloader. This is discuess in https://forums.truenas.com/t/how-to-install-jdownloader2/64746/11 and #4688

Testing

Tested locally with:

• basic-values.yaml

All tests passed successfully.

Checklist

• App runs successfully locally
• Only modified files under /ix-dev/ or /library/
• questions.yaml has clear descriptions and follows structure of existing apps
• All automated CI checks pass

[stavros-k]

thanks