docs: name 502/503/504 explicitly in the v1.0.1 retry notes (PR #48 review)

"a genuine transient 5xx" could be read to include 500, which is NOT retried. Say
"only a 502, 503, or 504" to match the code (TRANSIENT_STATUS) and the very next
sentence. Also tightened "500 / other 4xx" (500 isn't a 4xx) to "a 500, any other 4xx".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: ndemianc

RELEASE-NOTES.md

@@ -8,7 +8,7 @@ A reliability patch. A transient upstream hiccup now **retries and recovers** in
 
 When the model gateway briefly can't reach a healthy backend it returns a **502 / 503 / 504** — a momentary hiccup that used to end the whole run. LevelCode now **retries once, before anything has streamed**, so the common case (the backend is fine a second later) just recovers and your turn carries on. You'll see a brief `upstream busy — retrying…` instead of a dead run.
 
-It's deliberate about *when* it retries: only a genuine transient 5xx, and only before any output has appeared — so a retry can never duplicate text or double-charge. It does **not** retry a rate-limit (429), a real request error (500 / other 4xx), or an aborted request, and a 401 still refreshes your session as before. Hitting **Stop** during the wait stays instant.
+It's deliberate about *when* it retries: only a **502, 503, or 504**, and only before any output has appeared — so a retry can never duplicate text or double-charge. It does **not** retry a rate-limit (429), a 500, any other 4xx, or an aborted request, and a 401 still refreshes your session as before. Hitting **Stop** during the wait stays instant.
 
 ### Honest error messages