Clarify that credentials have to be "secret text" credentials

[ite-klass]

The configuration does not make clear that a secret text credential has to be used. Adding a descriptive text or expandable help text would be helpful.

This is especially confusing when the GitLab plugin is installed as well, which adds a credential category for personal API tokens. Given that this plugins setting is labeled as "personal API token" as well, it implies that that credential category should be used, but then fails to identify it. (Adding the credentials from the Add button and it then simply not appearing as a selectable option is quite confusing.)