For background on the docs in the kernel source tree, LWN (as is often the case) had a good article about it: https://lwn.net/Articles/692704/

There's also a copy on kernel. org, not sure why Intel's copy is being linked here. It's probably the same text, so any copy should be equally good, it just feels less canonical to me. Probably silliness on my part for even thinking of it.