Cgroups from Arch Wiki
wiki.archlinux.org
wiki.archlinux.org
However, in many cases its documentation is great for giving you basic directives how to do common tasks and troubleshooting esoteric problems, but doesn't give you any deep understanding of any tool, or its interactions with the rest of the system.
The common pattern I've noticed is:
<TOOL>
<TOOL> is a tool for doing blah blah...
If you want to do <X> with <TOOL>, run this command:
<COMMAND>
If you want to do <Y>, run this command:
<COMMAND>
[...]
Of course, if you're a user of Arch Linux, it is expected that you know what you're doing, or at least are willing to read the manual. But reading the manuals individually doesn't give you a high-level understanding of how things fit together. I think Arch Wiki pages could bring a lot of value by providing more high-level perspective.
As I see it, properly understanding the design of the tool should be documentation provided by the tool itself, since that keeps it in sync with changes.
I do think what would be useful is 1 article explaining the different bits of how Linux fits together (or just point to articles). I think something explaining the how the various composition points (e.g. filesystems, boot loaders,daemons and periodic jobs etc.) fit together would be quite useful.