doc: document the support status

Signed-off-by: Shiwei Zhang <shizh@microsoft.com>

internal/spec/artifact.go

@@ -25,6 +25,12 @@ const MediaTypeArtifactManifest = "application/vnd.oci.artifact.manifest.v1+json
 
 // Artifact describes an artifact manifest.
 // This structure provides `application/vnd.oci.artifact.manifest.v1+json` mediatype when marshalled to JSON.
+//
+// This manifest type was introduced in image-spec v1.1.0-rc1 and was removed in
+// image-spec v1.1.0-rc3. It is not part of the current image-spec and is kept
+// here for Go compatibility.
+//
+// Reference: https://github.com/opencontainers/image-spec/pull/999
 type Artifact struct {
 	// MediaType is the media type of the object this schema refers to.
 	MediaType string `json:"mediaType"`