Document RemoteIO as ios unit fallback

iOS provides three I/O (input/output) units. The vast majority of audio-unit applications use the Remote I/O unit, which connects to input and output audio hardware and provides low-latency access to individual incoming and outgoing audio sample values. For VoIP apps, the Voice-Processing I/O unit extends the Remote I/O unit by adding acoustic echo cancelation and other features. To send audio back to your application rather than to output audio hardware, use the Generic Output unit.

See https://developer.apple.com/library/content/documentation/MusicAudio/Conceptual/AudioUnitHostingGuide_iOS/UsingSpecificAudioUnits/UsingSpecificAudioUnits.html

src/coreaudio/mod.rs

@@ -158,6 +158,9 @@ impl Voice {
         }
 
         let au_type = if cfg!(target_os = "ios") {
+        	// The DefaultOutput unit isn't available in iOS unfortunately. RemoteIO is a sensible replacement.
+        	// See 
+        	// https://developer.apple.com/library/content/documentation/MusicAudio/Conceptual/AudioUnitHostingGuide_iOS/UsingSpecificAudioUnits/UsingSpecificAudioUnits.html
             coreaudio::audio_unit::IOType::RemoteIO
         } else {
             coreaudio::audio_unit::IOType::DefaultOutput