Document kernel versions affected by cgroup and squashfs filesystems

[ikaneshiro]

Description

It is evident that the interaction of cgroups memory limits and the squashfs filesystem implementations on older, but still in use, linux kernel versions. This has been noted in several issues and investigated in a write up posted in the HPCng slack:

• Singularity, cgroup memory.limits, mmaped strangeness singularity#5850
• Reading large h5 files in parallel under singularity under a cgroup with insufficient memory results in long delay/hang singularity#5800
• https://hpcng.slack.com/files/U47TWTU1L/F02ERQ8S0AZ/singularity-squashfs-oom-issue.pdf

We should generate documentation articulating the issue and listing kernel versions known to be problematic as a part of cgroups.rst page.

Once documentation is generated, we should include a comment on the related issues with the affected range to make the information easy to find.