GUACAMOLE-547: Document return value of credential handler.
GUACAMOLE-547: Document return value of credential handler.

apache/guacamole-server,mike-jumper/incubator-guacamole-server,apache/guacamole-server,mike-jumper/incubator-guacamole-server,mike-jumper/incubator-guacamole-server,mike-jumper/incubator-guacamole-server,glyptodon/guacamole-server,apache/guacamole-server,glyptodon/guacamole-server,glyptodon/guacamole-server