Add docstring to triggerchallenge
Add docstring to triggerchallenge

The docstring now also reflects the client_mode.

privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea