text: Title: [DOC] Clarify documentation of `RMSE` to avoid confusions regarding the implementation
  Body: Noticed discrepancy related to metric definition. RMSE is actually MSE, which makes sense as RMSE tends to be very unstable.
  
  Definiton: 
  ```python
  class RMSE(MultiHorizonMetric):
    """
    Root mean square error
    
    Defined as ``(y_pred - target)**2``
    """
    
    def __init__(self, reduction="sqrt-mean", **kwargs):
     super().__init__(reduction=reduction, **kwargs)
    
    def loss(self, y_pred: Dict[str, torch.Tensor], target):
      loss = torch.pow(self.to_prediction(y_pred) - target, 2)
      return loss
  ```
  source:
  https://github.com/jdb78/pytorch-forecasting/blob/68a0eb5f1701801142ce976fa50305b29507845a/pytorch_forecasting/metrics/point.py#L137C1-L149C20