docs(routes): tighten the outage comments and notes

Shorter comments, changelog and CURRENT-STATE text for the 502 outage
path; the CURRENT-STATE paragraph now also names the 429 exception and
which budget expiry answers busy and which answers 502. Text only.

changelog.d/1509-route-outage-unavailable.md

@@ -1,4 +1,3 @@
-- When the routing service is down (an error status, no answer or a non-JSON
-  body), voice route planning hears "Routing service unavailable" and the
-  Directions layer shows "Routing unavailable" instead of "no route found",
-  so neither blames the request (quanticsoul4772, #1509).
+- Routing outages report "Routing service unavailable" in voice route planning
+  and "Routing unavailable" in Directions instead of "no route found"
+  (quanticsoul4772, #1509).