spec/manifest: clarify relationship between urls and foreign layers

Previously, the specificiation incorrectly bound the fates of `urls` and
foreign layers. These are complementary but unrelated features, in that
the `urls` field may be populated for layers that aren't foreign. The
type of the layer only dictates the push behavior of the layer, rather
than involving where it came from.

For example, one may pull a foreign layer from a registry, but they may
not push it back to another registry. Conversely, a layer that has no
restrictions on push/pull behavior may be fetched via `urls` entries.

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

docs/spec/manifest-v2-2.md

@@ -31,8 +31,9 @@ the resources they reference:
 - `application/vnd.docker.distribution.manifest.v1+json`: schema1 (existing manifest format)
 - `application/vnd.docker.distribution.manifest.v2+json`: New image manifest format (schemaVersion = 2)
 - `application/vnd.docker.distribution.manifest.list.v2+json`: Manifest list, aka "fat manifest"
-- `application/vnd.docker.image.rootfs.diff.tar.gzip`: "Layer", as a gzipped tar
 - `application/vnd.docker.container.image.v1+json`: Container config JSON
+- `application/vnd.docker.image.rootfs.diff.tar.gzip`: "Layer", as a gzipped tar
+- `application/vnd.docker.image.rootfs.foreign.diff.tar.gzip`: "Layer", as a gzipped tar that should never be pushed
 
 ## Manifest List
 
@@ -203,6 +204,9 @@ image. It's the direct replacement for the schema-1 manifest.
 
         The MIME type of the referenced object. This should
         generally be `application/vnd.docker.image.rootfs.diff.tar.gzip`.
+        Layers of type
+        `application/vnd.docker.image.rootfs.foreign.diff.tar.gzip` may be
+        pulled from a remote location but they should never be pushed.
 
     - **`size`** *int*
 
@@ -218,11 +222,9 @@ image. It's the direct replacement for the schema-1 manifest.
 
     - **`urls`** *array*
 
-        For an ordinary layer, this is empty, and the layer contents can be
-        retrieved directly from the registry. For a layer with *`mediatype`* of
-        `application/vnd.docker.image.rootfs.foreign.diff.tar.gzip`, this
-        contains a non-empty list of URLs from which this object can be
-        downloaded.
+        Provides a list of URLs from which the content may be fetched. Content
+        should be verified against the `digest` and `size`. This field is
+        optional and uncommon.
 
 ## Example Image Manifest