More documentation around what mutable paths exist

[yeazelm]

Path to page with conent (do not include domain name):
content/en/os/1.13.x/concepts/restricted-filesystem/index.markdown

What content is incorrect?
I expected there to be some discussion about what is specifically stored outside of the immutable root filesystem. There is state such as logs and container images that would be useful to call out so a user can find them if they need them.

What should the content say or communicate?

I would like to see specific paths/locations for commonly asked for files. Logs comes to mind first, but it would be useful to call out /local and what exists in that path and when to expect it to be there or not (for instance, are these files always persisted or cleaned up during upgrades, etc)