docs: update hmac-auth body validation example to sign Digest header

docs/en/latest/plugins/hmac-auth.md

@@ -887,6 +887,10 @@ body = '{"name": "world"}'          # example request body
 # skew to prolong the validity within the advised security boundary
 gmt_time = datetime.now(timezone.utc).strftime('%a, %d %b %Y %H:%M:%S GMT')
 
+# create the SHA-256 digest of the request body and base64 encode it
+body_digest = hashlib.sha256(body.encode('utf-8')).digest()
+body_digest_base64 = base64.b64encode(body_digest).decode('utf-8')
+
 # construct the signing string (ordered)
 # the date and any subsequent custom headers should be lowercased and separated by a
 # single space character, i.e. `<key>:<space><value>`
@@ -895,23 +899,20 @@ signing_string = (
     f"{key_id}\n"
     f"{request_method} {request_path}\n"
     f"date: {gmt_time}\n"
+    f"digest: SHA-256={body_digest_base64}\n"
 )
 
 # create signature
 signature = hmac.new(secret_key, signing_string.encode('utf-8'), hashlib.sha256).digest()
 signature_base64 = base64.b64encode(signature).decode('utf-8')
 
-# create the SHA-256 digest of the request body and base64 encode it
-body_digest = hashlib.sha256(body.encode('utf-8')).digest()
-body_digest_base64 = base64.b64encode(body_digest).decode('utf-8')
-
 # construct the request headers
 headers = {
     "Date": gmt_time,
     "Digest": f"SHA-256={body_digest_base64}",
     "Authorization": (
-        f'Signature keyId="{key_id}",algorithm="hmac-sha256",'
-        f'headers="@request-target date",'
+        f'Signature keyId="{key_id}",algorithm="{algorithm}",'
+        f'headers="@request-target date digest",'
         f'signature="{signature_base64}"'
     )
 }
@@ -972,6 +973,10 @@ You should see an `HTTP/1.1 200 OK` response similar to the following:
 }
 ```
 
+:::note
+`validate_request_body` only checks that the `Digest` header matches the request body. To bind the body integrity to the HMAC signature and ensure end-to-end protection, you must also include the `Digest` header in the signed headers list (as shown above) or enforce it via the route-level `signed_headers` configuration option.
+:::
+
 If you send a request without the digest or with an invalid digest:
 
 ```shell