Add example of empty response and update wording

Signed-off-by: Lahiru Dissanayake <[email protected]>

manifest-referrers-api.md

@@ -47,7 +47,7 @@ In future versioned releases, responses MAY be extended to include a `data` fiel
 
 This paged result MUST return the following elements:
 
-- `references`: A list of [artifact descriptors][descriptor] that reference the given `subject`. The list MUST be empty if there are no artifacts referencing the given `subject`.
+- `references`: A list of [artifact descriptors][descriptor] that reference the given manifest. The list MUST be empty if there are no artifacts referencing the given manifest.
 
 **example result of artifacts that reference the `net-monitor` image:**
 ```json
@@ -69,6 +69,13 @@ This paged result MUST return the following elements:
 }
 ```
 
+**example result for a manifest that has no artifacts referencing it:**
+```json
+{
+  "references": []
+}
+```
+
 ### Paging Results
 
 The `/referrers` API MUST provide for paging, returning a list of [artifact descriptors](./descriptor.md).
@@ -153,7 +160,7 @@ GET /oras/artifacts/v1/net-monitor/manifests/sha256:3c3a4604a545cdc127456d94e421
 
 ### Error Handling
 
-If the `subject` identified by the digest does not exist in the repository, the registry MUST return a `404` response with the error code `MANIFEST_UNKNOWN`. 
+If the manifest does not exist, the registry MUST return a `404` response with the error code `MANIFEST_UNKNOWN`. 
 
 ## Further Reading