message: complete mysterious comments
diff: @@ -252,7 +252,7 @@ where
  fn intern_check(&self, db: &DB, key: &Q::Key) -> Option<StampedValue<InternId>> {
  let revision_now = db.salsa_runtime().current_revision();
  - // First,
  + // First, try with read lock -- this only works if `accessed_at` is up to date.
  {
  let tables = self.tables.read();
  let &index = tables.map.get(key)?;
  @@ -282,7 +282,7 @@ where
  }
  }
  - // Next,
  + // Acquire write lock if necessary.
  let mut tables = self.tables.write();
  let &index = tables.map.get(key)?;
  match &mut tables.values[index.as_usize()] {
  