message: update comment on `durability_last_changed_revision`
diff: @@ -158,7 +158,13 @@ where
  self.shared_state.revisions[0].load()
  }
  - /// The revision in which constants last changed.
  + /// The revision in which values with durability `d` may have last
  + /// changed. For D0, this is just the current revision. But for
  + /// higher levels of durability, this value may lag behind the
  + /// current revision. If we encounter a value of durability Di,
  + /// then, we can check this function to get a "bound" on when the
  + /// value may have changed, which allows us to skip walking its
  + /// dependencies.
  #[inline]
  pub(crate) fn durability_last_changed_revision(&self, d: Durability) -> Revision {
  self.shared_state.revisions[d.index()].load()
  