Clarifications around manifest reference validation

Be more specific for missing references in all manifests and
not just image manifests.

Signed-off-by: James Hewitt <james.hewitt@uk.ibm.com>

spec.md

@@ -190,9 +190,12 @@ If the blob or manifest is not found in the registry, the response code MUST be
 #### Push
 
 Pushing an artifact typically works in the opposite order as a pull: the blobs making up the artifact are uploaded first,
-and the manifest last. Strictly speaking, content can be uploaded to the registry in any order, but a registry MAY reject
-a manifest if it references blobs that are not yet uploaded, resulting in a `BLOB_UNKNOWN` error <sup>[code-1](#error-codes)</sup>.
-A useful diagram is provided [here](https://github.com/google/go-containerregistry/tree/d7f8d06c87ed209507dd5f2d723267fe35b38a9f/pkg/v1/remote#anatomy-of-an-image-upload).
+and the manifest last. A useful diagram is provided [here](https://github.com/google/go-containerregistry/tree/d7f8d06c87ed209507dd5f2d723267fe35b38a9f/pkg/v1/remote#anatomy-of-an-image-upload).
+
+A registry MAY reject an image manifest if it references blobs that are not yet uploaded, resulting in a `BLOB_UNKNOWN` error <sup>[code-1](#error-codes)</sup>.
+
+A registry MAY reject an image index if it references digests that do not exist in the registry, resulting in a `MANIFEST_BLOB_UNKNOWN` error <sup>[code-5](#error-codes)</sup>.
+
 
 ##### Pushing blobs