GUACAMOLE-422: Fix return documentation on opcode handler function.
GUACAMOLE-422: Fix return documentation on opcode handler function.

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