[Sparse] Polish the docstring of SparseMatrix.csr() and SparseMatrix.csc() (#5179)

* [Sparse] Polish the docstring of SparseMatrix.csr() and SparseMatrix.csc()

* Update sparse_matrix.py

* Update
Co-authored-by: Hongzhi (Steve), Chen <chenhongzhi.nkcs@gmail.com>

python/dgl/sparse/sparse_matrix.py

@@ -108,8 +108,11 @@ class SparseMatrix:
         Returns
         -------
         Tuple[torch.Tensor, torch.Tensor, torch.Tensor]
-            A tuple of tensors containing row, column coordinates and value
-            indices
+            A tuple of tensors containing row indptr, column indices and value
+            indices. Value indices is an index tensor, indicating the order of
+            the values of non-zero elements in the CSR representation. A null
+            value indices indicates the order of the values stays the same as
+            the values of the SparseMatrix.
         """
         return self.c_sparse_matrix.csr()
 
@@ -120,8 +123,11 @@ class SparseMatrix:
         Returns
         -------
         Tuple[torch.Tensor, torch.Tensor, torch.Tensor]
-            A tuple of tensors containing row, column coordinates and value
-            indices
+            A tuple of tensors containing column indptr, row indices and value
+            indices. Value indices is an index tensor, indicating the order of
+            the values of non-zero elements in the CSC representation. A null
+            value indices indicates the order of the values stays the same as
+            the values of the SparseMatrix.
         """
         return self.c_sparse_matrix.csc()