docs: clarify 429 honored-retry_after is additive with aretry backoff (F2)

note that an honored retry_after sleeps before aretry's own backoff, so the effective
wait is retry_after + backoff — only ever over-waits, never under-waits the server hint.

Signed-off-by: disqualifier <dev@disqualifier.me>

src/aiowebhooks/sender.py

@@ -214,7 +214,10 @@ class Webhook:
                     if status == 429:
                         # every 429 is retryable; honor an explicit retry_after by
                         # sleeping it, but a 429 with no parseable wait (edge/Cloudflare/
-                        # generic webhook) still retries under aretry's backoff + cap
+                        # generic webhook) still retries under aretry's backoff + cap.
+                        # note: aretry ALSO sleeps its backoff between retries, so an
+                        # honored retry_after is additive (retry_after + backoff) — this
+                        # only ever over-waits, never under-waits the server's hint
                         wait = self._retry_after(status, resp.headers, body)
                         if wait is not None:
                             log.warning("webhook 429 on %s; honoring retry_after %.3fs", url, wait)