DOC: config: use CREATE USER for mysql-check

DOC: config: use CREATE USER for mysql-check

CREATE USER has been the standard way of creating users since
MySQL-5.0 (2005).

The current syntax of INSERT INTO mysql.user won't actually work
on MariaDB-10.4+.

Because haproxy doesn't use any resources the MySQL executable comment
syntax provides resource contraints to make it more palatable
to risk adverse users.

/*!50701 is a syntax recognised by MySQL and MariaDB 5.7.1+ when
resource contraints where added.

/*M!100201 is a MariaDB executable comment syntax recognised for MariaDB
for the 10.2.1 where the MAX_STATEMENT_TIME was added.

This patch may be backported as far as 2.0.