docs: the retry/aretry decorator needs parentheses (@aretry())

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

README.md

@@ -202,6 +202,11 @@ async def pull():
     ...
 ```
 
+The decorator always needs parentheses, even with no kwargs: `@retry()` / `@aretry()`. A
+bare `@retry` / `@aretry` is the call form applied to the function: `@retry` runs it once
+at import time and replaces it with its return value, and `@aretry` replaces it with a
+coroutine that is never awaited.
+
 Knobs: `attempts` (total tries), `backoff` / `factor` / `max_backoff` (delay is
 `min(backoff * factor**n, max_backoff)`), `jitter` (full jitter, on by default),
 `on=` (tuple of retryable exception types), and `give_up=lambda exc: ...` to stop early