Revert to old wording pertaining to digests

Signed-off-by: Peter Engelbert <pmengelbert@gmail.com>

spec.md

@@ -333,6 +333,7 @@ The response to a successful mount MUST be `201 Created`, and MUST contain the f
 Location: <blob-location>
 ```
 
+The Location header will contain the registry URL to access the accepted layer file. The Docker-Content-Digest header returns the canonical digest of the uploaded blob which MAY differ from the provided digest. Most clients MAY ignore the value but if it is used, the client SHOULD verify the value against the uploaded blob data.
 
 Alternatively, if a registry does not support cross-repository mounting, it SHOULD return a `202`, indicating that the
 upload session has begun and that the client MAY proceed with the upload.