Add interface/NIC documentation to NBDE section

The RHEL Security Guide's section on using Network Bound Disk Encryption for Policy-Based Decryption (section 4.10.6) doesn't provide enough information for configuring static IP addressing in the dracut/initramfs. It is missing the information for choosing the interface (e.g. eth0, eth1, etc...)

I was able to use an alternative method, but it looks ugly.

echo 'kernel_cmdline="ip=192.168.122.254::192.168.122.1:255.255.255.0::eth1:none:192.168.122.1"' > /etc/dracut.conf.d/static-ip.conf

How can I specify the interface using your syntax, which is much prettier?

https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/security_guide/sec-policy-based_decryption#sec-Configuring-Manual-Enrollment