Alright, I added the following bit to the description of "bootloader --append=" in the list of KS commands and options:

>Among other things, this option is useful for disabling mechanisms which were implemented to mitigate the Meltdown and Spectre speculative execution vulnerabilities found in most modern processors. In some cases, these mechanisms may be unnecessary, and keeping them enabled would mean needlessly decreased performance. To disable these mechanisms, add their options into your Kickstart file - for example, bootloader --append="nopti noibrs noibpb" on AMD64/Intel 64 systems.
>
>=====
>Warning
>Before disabling any of the vulnerability mitigation mechanisms, make sure your system is not at risk of attack. See the [Red Hat vulnerability response article] for information about the Meltdown and Spectre vulnerabilities. 
>=====

I didn't include a full list of available options among architectures, and I didn't go into detail about what each option does. We should cover that in another document, either the Security Guide or the Performance Tuning Guide; I'm in contact with their owners. Once we have the list published somewhere, I'll add a link from the updated section in the install guide.

Verified by peer and SME on IRC.

Released with RHEL 7.5 GA documentation.