descriptor: clarify platform and artifacts

Signed-off-by: Bjorn Neergaard <bjorn.neergaard@docker.com>

descriptor.md

@@ -54,11 +54,13 @@ The following fields contain the primary properties that constitute a Descriptor
 
 - **`artifactType`** *string*
 
-  This OPTIONAL property contains the type of an artifact when the descriptor points to an [artifact](artifacts-guidance.md).
+  This OPTIONAL property contains the type of an artifact when the descriptor points to an [Artifact](artifacts-guidance.md).
   This property MUST be the same as the `artifactType` of the referenced [manifest](manifest.md), or the `mediaType` of the `config` descriptor if an `artifactType` is not set.
   If defined, the value MUST comply with [RFC 6838][rfc6838], including the [naming requirements in its section 4.2][rfc6838-s4.2], and MAY be registered with [IANA][iana].
 
-Descriptors pointing to [`application/vnd.oci.image.manifest.v1+json`](manifest.md) SHOULD include the extended field `platform`, see [Image Index Property Descriptions](image-index.md#image-index-property-descriptions) for details.
+Descriptors referencing an Image (as defined in the [artifacts guidance](artifacts-guidance.md)) SHOULD include the extended field `platform`.
+See [Image Index Property Descriptions](image-index.md#image-index-property-descriptions) for details.
+Artifacts as defined in the artifacts guidance MAY include the extended field `platform` when it improves discoverability or interoperability of the artifact. 
 
 ### Reserved