Clarify the layer selection behaviour

Co-authored-by: Hidde Beydals <hiddeco@users.noreply.github.com>
Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

rfcs/0003-kubernetes-oci/README.md

@@ -137,15 +137,24 @@ spec:
 ### Layer selection
 
 By default, Flux assumes that the first layer of the OCI artifact contains the Kubernetes configuration.
-For multi-layer artifacts created by other tools than Flux CLI, users can specify the media type of the layer
+For multi-layer artifacts created by other tools than Flux CLI
+(e.g. [oras](https://github.com/oras-project/oras),
+[crane](https://github.com/google/go-containerregistry/tree/main/cmd/crane)),
+users can specify the [media type](https://github.com/opencontainers/image-spec/blob/v1.0.2/media-types.md) of the layer
 which contains the tarball with Kubernetes manifests.
 
 ```yaml
 spec:
   layerSelector:
-    mediaType: "application/vnd.cncf.flux.content.v1.tar+gzip"
+    mediaType: "application/deployment.content.v1.tar+gzip"
 ```
 
+If the layer selector matches more than one layer,
+the first layer matching the specified media type will be used.
+Note that Flux requires that the OCI layer is
+[compressed](https://github.com/opencontainers/image-spec/blob/v1.0.2/layer.md#gzip-media-types)
+in the `tar+gzip` format.
+
 ### Pull artifacts from private repositories
 
 For authentication purposes, Flux users can choose between supplying static credentials with Kubernetes secrets