IPv4 only restriction wrong for IBM Z and IBM Power

Restriction "Only IPv4 addresses are supported" in the networking docs is not correct for IBM Z and IBM Power. 
see:  https://docs.openshift.com/container-platform/4.13/installing/installing_ibm_z/installing-restricted-networks-ibm-z.html#installation-configuration-parameters-network_installing-restricted-networks-ibm-z

Text should be the same as for example in bare metal: 

https://docs.openshift.com/container-platform/4.13/installing/installing_bare_metal/installing-bare-metal-network-customizations.html#installation-configuration-parameters-network_installing-bare-metal-network-customizations

need to discuss with multi-arch QE how far to backport

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