message: Document enter_shared function
diff: @@ -90,6 +90,10 @@ impl<'a> Visitor<'a> {
  }
  }
  + /// Enter a field containing a shared pointed.
  + ///
  + /// This functions does nothing and returns `None`
  + /// if pointee (`ptr` argument) was previously visited.
  pub fn enter_shared<'b>(
  &'b mut self,
  name: Key,
  