Add docs to custom vertex attribute example (#17422)

# Objective

The gltf-json crate seems like it strips/adds an `_` when doing the name
comparison for custom vertex attributes.

* gltf-json
[add](88e719d5de/gltf-json/src/mesh.rs (L341))
* gltf-json
[strip](88e719d5de/gltf-json/src/mesh.rs (L298C12-L298C42))
* [bevy's
handling](b66c3ceb0e/crates/bevy_gltf/src/vertex_attributes.rs (L273-L276))
seems like it uses the non-underscore'd version.

The bevy example gltf:
[barycentric.gltf](b66c3ceb0e/assets/models/barycentric/barycentric.gltf),
includes two underscores: `__BARYCENTRIC` in the gltf file, resulting in
needing `_BARYCENTRIC` (one underscore) as the attribute name in Bevy.
This extra underscore is redundant and does not appear if exporting from
blender, which only requires a single underscore to trigger the
attribute export.

I'm not sure if we want to change the example itself (maybe there's a
reason it has two underscores, I couldn't find a reason), but a docs
comment would help.

## Solution

add docs detailing the behavior

examples/2d/custom_gltf_vertex_attribute.rs

@@ -33,6 +33,9 @@ fn main() {
             DefaultPlugins.set(
                 GltfPlugin::default()
                     // Map a custom glTF attribute name to a `MeshVertexAttribute`.
+                    // The glTF file used here has an attribute name with *two*
+                    // underscores: __BARYCENTRIC
+                    // One is stripped to do the comparison here.
                     .add_custom_vertex_attribute("_BARYCENTRIC", ATTRIBUTE_BARYCENTRIC),
             ),
             Material2dPlugin::<CustomMaterial>::default(),