binlog_checksum dynamic changes seem ineffective, restart required

Hi,

I set up replication from MariaDB 10.3.30 slave to an old 5.5.29-29.4-log Percona Server master.

I got errors with the I/O thread:
```
Last_IO_Errno: 1595
Last_IO_Error: Relay log write failure: could not queue event from master
```

Some research indicated me to binlog_checksum now being enabled by default. The parameter is dynamic (according to the documentation and from being able to change it with SET GLOBAL) so I changed it that way. After I tried to fix replication by restarting the I/O thread and restarting replication entirely. Both were ineffective and generated the same error.

I added the parameter to the configuration file and restarted the server. Replication now came up without issues.

Please fix it by making the parameter dynamic again, or by making it not dynamic and changing the documentation.