[ir] Comment MultiplanarExternalTexture properly

Change-Id: Iabec3a7b6298b3d05c6cf52a87a5dc1601c12965
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/152560
Kokoro: Kokoro <noreply+kokoro@google.com>
Kokoro: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: James Price <jrprice@google.com>

src/tint/lang/core/ir/transform/multiplanar_external_texture.h

diff --git a/src/tint/lang/core/ir/transform/multiplanar_external_texture.h b/src/tint/lang/core/ir/transform/multiplanar_external_texture.h
index 780953b..2fc6dd9 100644
--- a/src/tint/lang/core/ir/transform/multiplanar_external_texture.h
+++ b/src/tint/lang/core/ir/transform/multiplanar_external_texture.h
@@ -27,7 +27,9 @@
 
 namespace tint::core::ir::transform {
 
-/// MultiplanarExternalTexture is a transform that... TODO
+/// MultiplanarExternalTexture is a transform that splits texture_external bindings into two
+/// separate texture_2d<f32> bindings for two possible planes, along with a uniform buffer of
+/// parameters that describe how the texture should be sampled.
 /// @param module the module to transform
 /// @param options the external texture options
 /// @returns an error string on failure