Add comment to custom vertex attribute example to make it easier to convert to 2D (#18603)

# Objective

- It's not clear what changes are needed to the shader to convert the
example to 2D.
- If you leave the shader unchanged you get a very confusing error (see
linked issue).
- Fixes #14077

## Solution

A separate example probably isn't needed as there is little difference
between 3D and 2D, but a note saying what changes are needed to the
shader would make it a lot easier.

Let me know if you think it is also worth adding some notes to the rust
file, but it is mostly trivial changes such as changing `Mesh3d` to
`Mesh2d`. I have left the original code in comments next to the changes
in the gist linked at the bottom if you wish to compare.

## Testing

- I just spent a long time working it out the hard way. This would have
made it a lot quicker.
- I have tested the 2D version of the shader with the changes explained
in the suggested comment and it works as expected.
- For testing purposes [here is a complete working 2D
example](https://gist.github.com/nickyfahey/647e2a2c45e695f24e288432b811dfc2).
(note that as per the original example the shader file needs to go in
'assets/shaders/')

assets/shaders/custom_vertex_attribute.wgsl

@@ -1,3 +1,5 @@
+// For 2d replace `bevy_pbr::mesh_functions` with `bevy_sprite::mesh2d_functions`
+// and `mesh_position_local_to_clip` with `mesh2d_position_local_to_clip`.
 #import bevy_pbr::mesh_functions::{get_world_from_local, mesh_position_local_to_clip}
 
 struct CustomMaterial {