[Doc] update neighbor sampler doc. (#287)

python/dgl/contrib/sampling/sampler.py

@@ -130,8 +130,9 @@ def NeighborSampler(g, batch_size, expand_factor, num_hops=1,
     
     Returns
     -------
-    A subgraph loader that returns a list of batched subgraphs and a dictionary of
-        additional information about the subgraphs.
+    A subgraph iterator
+        The iterator returns a list of batched subgraphs and a dictionary of additional
+        information about the subgraphs.
     '''
     return NSSubgraphLoader(g, batch_size, expand_factor, num_hops, neighbor_type, node_prob,
                             seed_nodes, shuffle, num_workers, max_subgraph_size, return_seed_id)