[Doc] fix a doc for partition_graph. (#1657)

* fix a doc for partition_graph.

* Update partition.py

* fix.

python/dgl/distributed/partition.py

@@ -180,7 +180,15 @@ def partition_graph(g, graph_name, num_parts, out_path, num_hops=1, part_method=
     stored in a dictionary, in which the key is the edge data name and the value is a tensor.
 
     The graph structure of a partition is stored in a file with the DGLGraph format. The DGLGraph
-    contains the mapping of node/edge Ids to the Ids in the original graph.
+    contains the mapping of node/edge Ids to the Ids in the global graph. The mappings can be
+    accessed with `part.ndata[dgl.NID]` and `part.edata[dgl.NID]`, where `part` is the partition
+    graph structure. In addition to the mapping, the partition graph contains node data
+    ("inner_node" and "orig_id") and edge data ("inner_edge").
+
+    * "inner_node" indicates whether a node belongs to a partition.
+    * "inner_edge" indicates whether an edge belongs to a partition.
+    * "orig_id" exists when reshuffle=True. It indicates the original node Ids in the original
+    graph before reshuffling.
 
     Parameters
     ----------