The admin.listeners config has wonky valid values in the docs

Noticed this while updating the docs for the 2.6.2 release, the docs for these configs are generated from the config definition, including info such as default, type, valid values, etc.

When defining WorkerConfig.ADMIN_LISTENERS_CONFIG we seem to pass an actual `new AdminListenersValidator()` object in for the "valid values" parameter, causing this field to display some wonky useless object reference in the docs. See https://kafka.apache.org/documentation/#connectconfigs_admin.listeners:

Valid Values: org.apache.kafka.connect.runtime.WorkerConfig$AdminListenersValidator@383534aa