Our customer forgot to add the "additionalTrustBundle" configuration during the install-config.yaml creation resulted in a failed installation.

This step is only mentioned in the "Configuring the Agent-based Installer to use mirrored images" section:
https://docs.openshift.com/container-platform/4.15/installing/installing_with_agent_based_installer/understanding-disconnected-installation-mirroring.html#configuring-the-agent-based-installer-to-use-mirrored-images

However, as the creation of the mirror registry and following the installation steps can take some time it will be nice to have this step reminded also during the creation of the install-config.yaml

An INFO-Note style box could be useful to remind that when using a disconnected mirror registry it will be required to add the certificate created in the previous step or pages.