Identify limitations in non-setuid mode

[dtrudg]

We need to clearly call out things that don't work, or behave differently in non-setuid and setuid but user-namespace mode.

This is a very common theme in issues posted to the main Sylabs GitHub repo.

[dtrudg]

The requirements and limitations for non-setuid / user namespace flows are now detailed fairly well in the updated admin guide. We reference the revamped install instructions in the admin guide, which call this out, from the user guide.

I think that to go further here we might need a convention of labels or icons around commands/options - so it's clear at a glance to users what doesn't work without setuid - rather than duplicating the admin guide explanatory text.

We can work on this going forward for 3.6 perhaps?