LP:520243 - useability bug of thread pool configuration

If I configure MariaDB 5.1.42 uses thread pool to handle the incoming connection as following:
thread-handling=pool-of-threads
thread-pool-size=20

I suppose I can check out these parameters by
mysqld --verbose --help

However, only "thread-pool-size" is displayed in the printout result. "thread-handling" doesn't show on the printout results.

BTW, if I type "SHOW VARIABLES" in mysql shell, "thread-handling" can be printed out in the result.