docs(playback): document native error evidence

.changes/playback-accurate-source-errors.md

@@ -0,0 +1,7 @@
+---
+type: fix
+area: playback
+issues: [1159]
+---
+
+Unavailable streams no longer appear as unsupported codecs. When Video.js exposes a server error such as HTTP 404, the player shows that status; otherwise ambiguous source errors remain unidentified instead of guessing.