Add generated documentation for image restrictions

[stgraber]

Incus is slowly moving to automatically generated documentation for its configuration keys.

This can be done through the gendoc:generate type comments in the code, followed by running make update-metadata and consumption in the documentation through {include} ../config-option.txt.

This issue is for the keys defined in doc/image-handling.md to be moved to code in internal/server/instance/drivers/driver_lxc.go and internal/server/instance/drivers/driver_qemu.go.

[TinkeringWithUS]

Can I get this assigned? We just to have the gen doc comment above where the parameter is being assigned right?

[stgraber]

Done!

[TinkeringWithUS]

Thanks! How can I validate that documentation is correct?

[TinkeringWithUS]

The defaults in the documentation is a hyphen, -. In the gendoc comment, would it be better for the default to be specified as a blank like so "default: " or just not leave out the default field entirely? And to clarify, since there's no validation section for image requirements, I'm putting the gendoc comments above where the requirements are being checked.
// gendoc:generate(entity=image, group=restrictions, key=privileged) .. if util.IsFalse(d.localConfig["image.requirements.privileged"]) && util.IsTrue(d.expandedConfig["security.privileged"]) { return "", nil, fmt.Errorf("The image used by this instance is incompatible with privileged containers. Please unset security.privileged on the instance") }. Is this fine?

[stgraber]

I think we've usually just left out the default value in such cases.

And yeah, right above the actual check is fine for keys like those which don't have a validator.

[TinkeringWithUS]

Is this current format fine for image requirements? For comparison, the current key display.