Document that this doesn't work for instances of a type #133

I got very excited about using this, and then realised after my attempt that it can only dispatch on types, not instances of types.

I have an enum type in https://github.com/google/osv.dev/blob/79c07c93dc01ed256032cb790eaae638a4255ee1/osv/models.py#L783 and had wanted to extensibly dispatch to different types, rather than what's currently going on in https://github.com/google/osv.dev/blob/79c07c93dc01ed256032cb790eaae638a4255ee1/docker/importer/importer.py#L519