docs: refresh tailscale auth rate limit refs

docs/help/troubleshooting.md

@@ -142,6 +142,9 @@ flowchart TD
     - That cached-token retry reuses the cached scope set stored with the paired
       device token. Explicit `deviceToken` / explicit `scopes` callers keep
       their requested scope set instead.
+    - On the async Tailscale Serve Control UI path, failed attempts for the same
+      `{scope, ip}` are serialized before the limiter records the failure, so a
+      second concurrent bad retry can already show `retry later`.
     - `too many failed authentication attempts (retry later)` from a localhost
       browser origin → repeated failures from that same `Origin` are temporarily
       locked out; another localhost origin uses a separate bucket.