message: docify some comments
diff: @@ -123,15 +123,15 @@ impl fmt::Display for MiriMemoryKind {
  #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
  pub struct Tag {
  pub alloc_id: AllocId,
  - // Stacked Borrows tag.
  + /// Stacked Borrows tag.
  pub sb: SbTag,
  }
  impl Provenance for Tag {
  - // We use absolute addresses in the `offset` of a `Pointer<Tag>`.
  + /// We use absolute addresses in the `offset` of a `Pointer<Tag>`.
  const OFFSET_IS_ADDR: bool = true;
  - // We cannot err on partial overwrites, it happens too often in practice (due to unions).
  + /// We cannot err on partial overwrites, it happens too often in practice (due to unions).
  const ERR_ON_PARTIAL_PTR_OVERWRITE: bool = false;
  fn fmt(ptr: &Pointer<Self>, f: &mut fmt::Formatter<'_>) -> fmt::Result {
  