In the current official documentation of IPI installation of Openstack [1]. 

[1]: https://docs.openshift.com/container-platform/4.14/installing/installing_openstack/installing-openstack-installer-custom.html 

The use of additional Network: 
The documentation just indicates the entry in install-config.yaml to enable additional networks, like this:

~~~
You can add additional networks, including provider networks, to the platform.openstack.additionalNetworkIDs list.After you deploy your cluster, you can attach pods to additional networks. For more information, see Understanding multiple networks.

[+] https://docs.openshift.com/container-platform/4.14/installing/installing_openstack/installing-openstack-installer-custom.html#:~:text=You%20can%20add,multiple%20networks.
~~~

However, the default OpenStack behavior is to enable DHCP to makes it more versatile and dynamic. 

It is observed that openshift installation with multiple network fails if the additional network are DHCP enabled.

Hence below information needs to be updated in the documentation.

"Make sure to disable DHCP on the additional network as it leads to failure of the cluster installation".  
[+] https://docs.openshift.com/container-platform/4.14/installing/installing_openstack/installing-openstack-installer-custom.html#:~:text=You%20can%20add,multiple%20networks. 

    4.14.z

    1.
    2.
    3.
    

   Cluster Installation fails with additional Network 

    Installation should be successful