Add a few missing doc comments in bevy_image (#19493)

# Objective

Another tiny step towards
https://github.com/bevyengine/bevy/issues/3492.

---------

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

crates/bevy_image/src/dynamic_texture_atlas_builder.rs

@@ -5,8 +5,11 @@ use guillotiere::{size2, Allocation, AtlasAllocator};
 use thiserror::Error;
 use tracing::error;
 
+/// An error produced by [`DynamicTextureAtlasBuilder`] when trying to add a new
+/// texture to a [`TextureAtlasLayout`].
 #[derive(Debug, Error)]
 pub enum DynamicTextureAtlasBuilderError {
+    /// Unable to allocate space within the atlas for the new texture
     #[error("Couldn't allocate space to add the image requested")]
     FailedToAllocateSpace,
     /// Attempted to add a texture to an uninitialized atlas

crates/bevy_image/src/image.rs

@@ -22,7 +22,10 @@ use wgpu_types::{
     TextureViewDescriptor,
 };
 
+/// Trait used to provide default values for Bevy-external types that
+/// do not implement [`Default`].
 pub trait BevyDefault {
+    /// Returns the default value for a type.
     fn bevy_default() -> Self;
 }