manifest: use this scratch descriptor for the SCRATCH layer

Ref: opencontainers#1025

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

manifest.md

@@ -46,7 +46,7 @@ Unlike the [image index](image-index.md), which contains information about a set
     While an empty blob (`size` of 0) may be preferable, practice has shown that not to be ubiquitiously supported.
     Instead, the blob payload can be the most minimal content that is still valid JSON object: `{}` (`size` of 2).
     The blob digest of `{}` is `sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a`.
-    See the [example SCRATCH config](#example-of-a-scratch-config-descriptor) below.
+    See the [example SCRATCH config](#example-of-a-scratch-config-or-layer-descriptor) below, and the `ScratchDescriptor()` of the reference code.
 
 - **`layers`** *array of objects*
 
@@ -56,6 +56,9 @@ Unlike the [image index](image-index.md), which contains information about a set
     The final filesystem layout MUST match the result of [applying](layer.md#applying-changesets) the layers to an empty directory.
     The [ownership, mode, and other attributes](layer.md#file-attributes) of the initial empty directory are unspecified.
 
+    For broad portability, if a layer is required to be used, use the SCRATCH layer.
+    See the [example SCRATCH layer](#example-of-a-scratch-config-or-layer-descriptor) below, and the `ScratchDescriptor()` of the reference code.
+
     Beyond the [descriptor requirements](descriptor.md#properties), the value has the following additional restrictions:
 
     - **`mediaType`** *string*
@@ -128,7 +131,7 @@ Unlike the [image index](image-index.md), which contains information about a set
 }
 ```
 
-## Example of a SCRATCH config descriptor
+## Example of a SCRATCH config or layer descriptor
 
 ```json,title=SCRATCH%20config&mediatype=application/vnd.oci.descriptor.v1%2Bjson
 {