updated audio_source.rs documentation (#12765)

# Objective
- Fixes #12677 

## Solution
Updated documentation to make it explicit that enabling the appropriate
optional features is required to use the supported audio file format, as
well as provided link to the Bevy docs listing the optional features.

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

crates/bevy_audio/src/audio_source.rs

@@ -11,8 +11,10 @@ pub struct AudioSource {
     /// Raw data of the audio source.
     ///
     /// The data must be one of the file formats supported by Bevy (`wav`, `ogg`, `flac`, or `mp3`).
-    /// It is decoded using [`rodio::decoder::Decoder`](https://docs.rs/rodio/latest/rodio/decoder/struct.Decoder.html).
+    /// However, support for these file formats is not part of Bevy's [`default feature set`](https://docs.rs/bevy/latest/bevy/index.html#default-features).
+    /// In order to be able to use these file formats, you will have to enable the appropriate [`optional features`](https://docs.rs/bevy/latest/bevy/index.html#optional-features).
     ///
+    /// It is decoded using [`rodio::decoder::Decoder`](https://docs.rs/rodio/latest/rodio/decoder/struct.Decoder.html).
     /// The decoder has conditionally compiled methods
     /// depending on the features enabled.
     /// If the format used is not enabled,