Document missing Opus encoder error

And how to solve it.

doc/audio.md

@@ -35,6 +35,13 @@ scrcpy --audio-codec=aac
 scrcpy --audio-codec=raw
 ```
 
+In particular, if you get the following error:
+
+> Failed to initialize audio/opus, error 0xfffffffe
+
+then your device has no Opus encoder: try `scrcpy --audio-codec=aac`.
+
+
 Several encoders may be available on the device. They can be listed by:
 
 ```bash