[wasm-gc][Subtyping] Update comment about heap type hierarchies

Bug: v8:7748
Change-Id: I678d890cf5a6a606790d782eb4cbc8b574faf4d5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3913399
Auto-Submit: Matthias Liedtke <mliedtke@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83400}

src/wasm/wasm-subtyping.h

@@ -47,19 +47,26 @@ V8_NOINLINE V8_EXPORT_PRIVATE bool EquivalentTypes(ValueType type1,
 // - (ref ht1) <: (ref null? ht2) iff ht1 <: ht2.
 // - rtt1 <: rtt2 iff rtt1 ~ rtt2.
 // For heap types, the following subtyping rules hold:
-// - The abstract heap types form the following type hierarchy:
-//      any (a.k.a. extern)
-//            /   \
-//           eq   func
+// - The abstract heap types form the following type hierarchies:
+//   TODO(7748): abstract ref.data should become ref.struct.
+//
+//           any              func         extern
+//            |                |             |
+//           eq              nofunc       noextern
 //          /  \
 //        i31   data
-//               |
-//             array
+//         |     |
+//         |   array
+//          \   /
+//          none
+//
 // - All functions are subtypes of func.
 // - All structs are subtypes of data.
 // - All arrays are subtypes of array.
 // - An indexed heap type h1 is a subtype of indexed heap type h2 if h2 is
 //   transitively an explicit canonical supertype of h1.
+// Note that {any} includes references introduced by the host which belong to
+// none of any's subtypes (e.g. JS objects).
 V8_INLINE bool IsSubtypeOf(ValueType subtype, ValueType supertype,
                            const WasmModule* sub_module,
                            const WasmModule* super_module) {