Inconsistent use of container and instance in instance configuration devices documentation

[kadinsayani]

The nouns container and instance are used interchangeably and inconsistently for the following device types:

unix-char
unix-block
usb
unix-hotplug

The usb device type is supported for both containers and VM's, implying that instance should be used when describing the device options. However, container is used for options such as gid, uid, and mode where the container condition is present.

The unix-char, unix-block, and unix-hotplug device types are only supported for containers, implying that container should be used when describing the device options. However, the word instance is used for all device options.

I think the best approach is to consistently use container for devices that are only supported for containers, and use instance for devices that are supported by both containers and VMs. For device options with the container condition present, the word instance should still be used.

[github-actions]

Heads up @mionaalex - the "Documentation" label was applied to this issue.