dev(MeanIoU): refine docstrings

mmeval/segmentation/mean_iou.py

@@ -20,9 +20,9 @@ class MeanIoU(BaseMetric):
  In addition to mean iou, it will also compute and return accuracy, mean
  accuracy, mean dice, mean precision, mean recall and mean f-score.
 
- Currently, there are 2 implementations of this metric: NumPy and PyTorch.
- Which implementation to use is determined by the type of the calling
- parameters. e.g. `numpy.ndarray` or `torch.Tensor`.
+ This metric supports 2 kinds of inputs, i.e. ``numpy.ndarray`` and
+ ``torch.Tensor``, and the implementation for the calculation depends on
+ the inputs type.
 
  Args:
  num_classes (int, optional): The number of classes. If None, it will be