message: Document struct hash/equality inconsistency
diff: @@ -138,6 +138,11 @@ where
  }
  fn write_hash(&self, hasher: &mut StarlarkHasher) -> anyhow::Result<()> {
  + // TODO(nga): hash and equality are inconsistent:
  + // Hash is ordered, equality ignores order.
  + // So structs like `struct(a=1, b=2)` and `struct(b=2, a=1)`
  + // are equal but have different hashes.
  +
  for (k, v) in self.fields.iter_hashed() {
  Hash::hash(&k, hasher);
  v.write_hash(hasher)?;
  