`TextureAtlas::texture_handles` field is not docuemented.

## How can Bevy's documentation be improved?

https://dev-docs.bevyengine.org/bevy/sprite/struct.TextureAtlas.html

I was hoping that I could use this to get a set of handles to each texture atlas image. Unfortunately, I do not appear to be able to extract this information, as the value was `None`.

The use of this field is not documented, nor are the situations in which it may be `Some`.