Update migration guide

Author: Waridley

release-content/migration-guides/mesh_compute_smooth_normals.md

@@ -3,27 +3,45 @@ title: Smooth normals implementation changed
 pull_requests: [18552]
 ---
 
-In Bevy 0.16, `Mesh` smooth normal calculation used a face area-weighted
-algorithm. In 0.17, the area-weighted method was moved to separate methods,
+In Bevy 0.16, `Mesh` smooth normal calculation used a triangle area-weighted
+algorithm. In 0.17, the area-weighted algorithm was moved to separate methods,
 the default implementation was switched to a corner angle-weighted algorithm,
 and `Mesh::compute_custom_smooth_normals` was added for other cases.
 
 The angle-weighted method is more suitable for growing or shrinking a mesh along
-its vertex normals, such as for generating an outline mesh. It also results in
-more expected lighting behavior in many cases. In most cases, the difference
+its vertex normals, such as when generating an outline mesh. It also results in
+more expected lighting behavior for some meshes. In most cases, the difference
 will be small and no change is needed. However, the new default is somewhat
 slower, and does not always produce the result desired by an artist. If you
-preferred the lighting in 0.16, have a significant performance regression,
-or needed face-weighted normals for any other reason, you can switch to the
-new dedicated face-weighted methods.
+preferred the lighting in 0.16, or have a significant performance regression,
+or needed area-weighted normals for any other reason, you can switch to the
+new dedicated area-weighted methods.
 
 ```diff
 // Only if the new smooth normals algorithm is unsatisfactory:
 
 let mut mesh = Mesh::new(PrimitiveTopology::TriangleList, default())
+    .with_inserted_attribute(Mesh::ATTRIBUTE_POSITION, positions)
 -    .with_computed_smooth_normals();
-+    .with_computed_face_weighted_normals;
++    .with_computed_area_weighted_normals;
 
 - mesh.compute_smooth_normals();
-+ mesh.compute_face_weighted_normals;
++ mesh.compute_area_weighted_normals();
+```
+
+As part of this change, the helper functions `face_normal` and
+`face_area_normal`, were renamed to `triangle_normal` and `triangle_area_normal`
+respectively to better reflect the fact that they do not take an entire
+geometric face into account.
+
+```diff
+- use bevy::render::mesh::face_normal;
+- let normal = face_normal(a, b, c);
++ use bevy::render::mesh::triangle_normal;
++ let normal = triangle_normal(a, b, c);
+
+- use bevy::render::mesh::face_area_normal;
+- let normal = face_area_normal(a, b, c);
++ use bevy::render::mesh::triangle_area_normal;
++ let normal = triangle_area_normal(a, b, c);
 ```