Add documentation for jackctl_server_add_slave, jackctl_server_remove_slave...

Add documentation for jackctl_server_add_slave, jackctl_server_remove_slave and jackctl_server_switch_master functions.

git-svn-id: http://subversion.jackaudio.org/jack/jack2/trunk/jackmp@3520 0c269be4-1314-0410-8aa9-9f06e86f4224