Clarify `mediaType` handling

Summary

### Impact In the OCI Image Specification version 1.0.1 and prior, manifest and index documents are not self-describing and documents with a single digest could be interpreted as either a manifest or an index. ### Patches The Image Specification will be updated to recommend that both manifest and index documents contain a `mediaType` field to identify the type of document. Release [v1.0.2](https://github.com/opencontainers/image-spec/releases/tag/v1.0.2) includes these updates. ### Workarounds Software attempting to deserialize an ambiguous document may reject the document if it contains both “manifests” and “layers” fields or “manifests” and “config” fields. ### References https://github.com/opencontainers/distribution-spec/security/advisories/GHSA-mc8v-mgrf-8f4m ### For more information If you have any questions or comments about this advisory: * Open an issue in https://github.com/opencontainers/image-spec * Email us at [[email protected]](mailto:[email protected]) * https://github.com/opencontainers/image-spec/commits/v1.0.2