Remove TODO and add docs about limitations of PlaybackMode::Once (#…

…16769)

# Objective

Fixes #12359

## Solution

Implement alternative number 4.

#12359 (comment)
> I don't think that I agree with the premise of this issue anymore. I
am not sure that entities "magically" despawning themselves or
components removing themselves make for great defaults in an "ECS-based
API". This behavior is likely to be just as surprising to people.
>
> I think that the lack of sink re-usability should be treated as a bug
and possibly the documentation improved to reflect the current
limitations if it doesn't seem like a fix is forthcoming.
> -- me

crates/bevy_audio/src/audio.rs

@@ -36,6 +36,12 @@ impl Volume {
 #[derive(Debug, Clone, Copy, Reflect)]
 pub enum PlaybackMode {
     /// Play the sound once. Do nothing when it ends.
+    ///
+    /// Note: It is not possible to reuse an `AudioPlayer` after it has finished playing and
+    /// the underlying `AudioSink` or `SpatialAudioSink` has been drained.
+    ///
+    /// To replay a sound, the audio components provided by `AudioPlayer` must be removed and
+    /// added again.
     Once,
     /// Repeat the sound forever.
     Loop,
@@ -77,13 +83,18 @@ pub struct PlaybackSettings {
 
 impl Default for PlaybackSettings {
     fn default() -> Self {
-        // TODO: what should the default be: ONCE/DESPAWN/REMOVE?
         Self::ONCE
     }
 }
 
 impl PlaybackSettings {
     /// Will play the associated audio source once.
+    ///
+    /// Note: It is not possible to reuse an `AudioPlayer` after it has finished playing and
+    /// the underlying `AudioSink` or `SpatialAudioSink` has been drained.
+    ///
+    /// To replay a sound, the audio components provided by `AudioPlayer` must be removed and
+    /// added again.
     pub const ONCE: PlaybackSettings = PlaybackSettings {
         mode: PlaybackMode::Once,
         volume: Volume(1.0),