Fatal error: Please read "Security" section of the manual to find out how to run mysqld as root!

When MariaDB is run under the root user, it fails with an error: Fatal error: Please read "Security" section of the manual to find out how to run mysqld as root!

This was written many years ago and was referring to the following paragraph in MySQL manual:

Never run the MySQL server as the Unix root user. This is extremely dangerous, because any user with the FILE privilege is able to cause the server to create files as root (for example, ~root/.bashrc). To prevent this, mysqld refuses to run as root unless that is specified explicitly using the --user=root option.

But we don't have that in the Knowledge Base. So this error leads user nowhere.

KB needs to explain how to run mysqld as root.