Add LastErrorMessage to /acl/replication docs (#11990)

website/content/api-docs/acl/index.mdx

@@ -128,7 +128,8 @@ $ curl \
   "ReplicatedIndex": 1976,
   "ReplicatedTokenIndex": 2018,
   "LastSuccess": "2018-11-03T06:28:58Z",
-  "LastError": "2016-11-03T06:28:28Z"
+  "LastError": "2016-11-03T06:28:28Z",
+  "LastErrorMessage": "failed to retrieve ACL policy updates: RPC rate limit exceeded"
 }
 ```
 
@@ -179,6 +180,9 @@ $ curl \
   replication process is not in a good state. A zero value of
   "0001-01-01T00:00:00Z" will be present if no sync has resulted in an error.
 
+- `LastErrorMessage` - The last error message produced at the time of `LastError`.
+  An empty string indicates that no sync has resulted in an error.
+
 ## Translate Rules
 
 -> **Deprecated** - This endpoint was removed in Consul 1.11.0.