writer/hlsl: Add some comments to pack_level_in_coords

This behavior was confusing in an earlier code review.

Change-Id: I1a8551d890f98d40e25f02d69bff88cf0c2356ac
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/56542
Auto-Submit: Ben Clayton <bclayton@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Ben Clayton <bclayton@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/writer/hlsl/generator_impl.cc

@@ -1663,7 +1663,12 @@ bool GeneratorImpl::EmitTextureCall(std::ostream& out,
   if (!EmitExpression(out, texture))
     return false;
 
+  // If pack_level_in_coords is true, then the mip level will be appended as the
+  // last value of the coordinates argument. If the WGSL intrinsic overload does
+  // not have a level parameter and pack_level_in_coords is true, then a zero
+  // mip level will be inserted.
   bool pack_level_in_coords = false;
+
   uint32_t hlsl_ret_width = 4u;
 
   switch (intrinsic->Type()) {
@@ -1689,6 +1694,7 @@ bool GeneratorImpl::EmitTextureCall(std::ostream& out,
       break;
     case sem::IntrinsicType::kTextureLoad:
       out << ".Load(";
+      // Multisampled textures do not support mip-levels.
       if (!texture_type->Is<sem::MultisampledTexture>()) {
         pack_level_in_coords = true;
       }