improve doc

README.md

@@ -218,6 +218,7 @@ tensor([[1, 2, 0, 3, 0, 3, 1, 2],
 ### Nearest
 
 Clusters points in *x* together which are nearest to a given query point in *y*.
+`batch_{x,y}` vectors need to be sorted.
 
 ```python
 import torch

torch_cluster/nearest.py

@@ -17,10 +17,12 @@ def nearest(x: torch.Tensor, y: torch.Tensor,
             :math:`\mathbf{Y} \in \mathbb{R}^{M \times F}`.
         batch_x (LongTensor, optional): Batch vector
             :math:`\mathbf{b} \in {\{ 0, \ldots, B-1\}}^N`, which assigns each
-            node to a specific example. (default: :obj:`None`)
+            node to a specific example. :obj:`batch_x` needs to be sorted.
+            (default: :obj:`None`)
         batch_y (LongTensor, optional): Batch vector
             :math:`\mathbf{b} \in {\{ 0, \ldots, B-1\}}^M`, which assigns each
-            node to a specific example. (default: :obj:`None`)
+            node to a specific example. :obj:`batch_y` needs to be sorted.
+            (default: :obj:`None`)
 
     :rtype: :class:`LongTensor`