image-layout.md, annotations.md: restrict scope of ref.name

By restricting the scope of the annotation
`org.opencontainers.image.ref.name` to only image indexes appearing in
the context of an image layout, we can greatly simplify processing and
selection of target content. While other properties may apply down, we
ensure that naming is restricted to a single "entity", the `index.json`,
ambiguity for a given reference is reduced.

This does not apply to other annotations, which may "apply downward" to
other resources referenced. The application of annotation downward is
specific to a particular annotation.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

annotations.md

@@ -22,4 +22,4 @@ This specification defines the following annotation keys, intended for but not l
 * **org.opencontainers.image.homepage** URL to find more information on the image (string, a URL with scheme HTTP or HTTPS)
 * **org.opencontainers.image.documentation** URL to get documentation on the image (string, a URL with scheme HTTP or HTTPS)
 * **org.opencontainers.image.source** URL to get source code for the binary files in the image (string, a URL with scheme HTTP or HTTPS)
-* **org.opencontainers.image.ref.name** Name of the reference (string)
+* **org.opencontainers.image.ref.name** Name of the reference for a target (string). Only valid when on index.json in [image layout](image-layout.md).