[install] rdo: adds explanation on existing cnf

In RHEL and CentOS distribution, mariadb-server.cnf file
exists by default MariaDB installation so let's add
brief explanation on existing configuration files.

Change-Id: I845bfa1119c25d3c9dce1c7a24bc79cb1518198c
Closes-Bug: #1745429

doc/install-guide/source/environment-sql-database-rdo.rst

@@ -19,6 +19,7 @@ Install and configure components
    .. end
 
 2. Create and edit the ``/etc/my.cnf.d/openstack.cnf`` file
+   (backup existing configuration files in ``/etc/my.cnf.d/`` if needed)
    and complete the following actions:
 
    - Create a ``[mysqld]`` section, and set the ``bind-address``