change documentation for --ftwrl-wait-timeout

The documentation here https://mariadb.com/kb/en/mariabackup-options/#-ftwrl-wait-timeout states, that --ftwrl-wait-tiomeout option behaves like the following: "If it encounters statements still in the process of executing, it waits until they complete before setting the lock." The fix for MDEV-20230 extends this functionality. Now it also finish mariabackup execution if backup lock(for the versions <10.4 this is FTWRL, for the versions >=10.4 this is "BACKUP STAGE BLOCK_COMMIT") can not be grubbed for the number of seconds pointed in this option.