docs(server): tighten compression comments and docs

changelog.d/795-opensky-compress-cached-body.md

@@ -1,7 +1,5 @@
-- The live flight snapshot is compressed on the wire. `/api/flights` gzips its
-  cached global snapshot and its regional adsb.lol fallback once per upstream
-  refresh and serves that frame to clients offering gzip — roughly 860 KB
-  becomes roughly 258 KB — while a client that offers nothing receives the same
-  bytes uncompressed. Both answers carry `Vary: Accept-Encoding`, and a response
-  the proxy does not cache, such as an auth failure, is never sent under a
-  cached snapshot's encoding (daikaginza, #795).
+- Compress the live flight snapshot on the wire. `/api/flights` gzips its
+  cached global snapshot and its regional adsb.lol fallback once per refresh
+  and sends that frame to clients that offer gzip: about 860 KB becomes about
+  250 KB. Other clients get the same bytes uncompressed, and both answers carry
+  `Vary: Accept-Encoding` (daikaginza, #795).