audio/ble_audio.h: add missing doxygen for ble_audio_broadcast_name

`ble_audio_broadcast_name` was not documented.

nimble/host/audio/include/audio/ble_audio.h

@@ -494,6 +494,7 @@ struct ble_audio_pub_broadcast_announcement {
     const uint8_t *metadata;
 };
 
+/** Broadcast Name */
 struct ble_audio_broadcast_name {
     /** Broadcast Name length */
     uint8_t name_len;