nixos/oci-containers: improve description of imageFile

As a novice to using this module, I found the existing description to be
quite misleading. It does not at all disable pulling from the registry,
it just loads some image archive that may or may not be related to the
container you're specifying. I had thought there was extra magic behind
this option, but it's just a `docker load`. You need foreknowledge of
the contents of the archive so that whatever it contained is actually
used to run the container.

I've reworded the description to hopefully make this behavior clearer.

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