Dds transcode channel order fix (#19849)

# Objective

- Fix bug in dds channel order transcode
- taken from #18411

## Solution

- fix it
- improve docs a bit

## Testing

- example in linked pr

crates/bevy_image/src/dds.rs

@@ -23,9 +23,9 @@ pub fn dds_buffer_to_image(
         Ok(format) => (format, None),
         Err(TextureError::FormatRequiresTranscodingError(TranscodeFormat::Rgb8)) => {
             let format = if is_srgb {
-                TextureFormat::Bgra8UnormSrgb
+                TextureFormat::Rgba8UnormSrgb
             } else {
-                TextureFormat::Bgra8Unorm
+                TextureFormat::Rgba8Unorm
             };
             (format, Some(TranscodeFormat::Rgb8))
         }

crates/bevy_image/src/image.rs

@@ -1558,11 +1558,11 @@ pub enum DataFormat {
 pub enum TranscodeFormat {
     Etc1s,
     Uastc(DataFormat),
-    // Has to be transcoded to R8Unorm for use with `wgpu`.
+    /// Has to be transcoded from `R8UnormSrgb` to `R8Unorm` for use with `wgpu`.
     R8UnormSrgb,
-    // Has to be transcoded to R8G8Unorm for use with `wgpu`.
+    /// Has to be transcoded from `Rg8UnormSrgb` to `R8G8Unorm` for use with `wgpu`.
     Rg8UnormSrgb,
-    // Has to be transcoded to Rgba8 for use with `wgpu`.
+    /// Has to be transcoded from `Rgb8` to `Rgba8` for use with `wgpu`.
     Rgb8,
 }