descriptor: clarify platform and artifacts

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

descriptor.md

@@ -58,7 +58,7 @@ The following fields contain the primary properties that constitute a Descriptor
   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