Document when `Camera::viewport_to_world` and related methods return `None`

## How can Bevy's documentation be improved?

Document when [`viewport_to_world`](https://docs.rs/bevy/latest/bevy/render/camera/struct.Camera.html#method.viewport_to_world) and related methods return `None`.