docs: Clarify filesystem support for user-namespace Pods

[utam0k]

Description

The user namespace with a pod has a limitation on the filesystem. This PR makes it clear.
ref: kubernetes/enhancements#127 (comment)

Issue

ref: kubernetes/enhancements#127 (comment)

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[lmktfy]

@kubernetes/sig-node-pr-reviews is this change technically accurate?

[rata]

The PR is accurate. I wonder if being more verbose, as I've just written here is better: https://kubernetes.io/blog/2025/04/25/userns-enabled-by-default/#everything-you-wanted-to-know-about-user-namespaces-in-kubernetes. See questions 3 and 4.

@utam0k what do you think?

[utam0k]

@rata Thanks for your suggestion. In my opinion, the user document has to keep it simple. How about adding the link as a reference for those who want to know more technical details?

[rata]

SGTM, thanks!

[rata]

LGTM, thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: rata
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[utam0k]

@lmktfy What should I do next?