[Doc] Computing with DGLGraph (#212)

* Docs for graph computing

* Docs for graph computing

* Fix

* WIP

* update the docstring

docs/source/api/python/udf.rst

@@ -4,6 +4,16 @@ User-defined function related data structures
 ==================================================
 
 .. currentmodule:: dgl.udf
+.. automodule:: dgl.udf
+
+There are two types of user-defined functions in DGL:
+
+* **Node UDF** of signature ``NodeBatch -> dict``. The argument represents
+  a batch of nodes. The returned dictionary should have ``str`` type key and ``tensor``
+  type values.
+* **Edge UDF** of signature ``EdgeBatch -> dict``. The argument represents
+  a batch of edges. The returned dictionary should have ``str`` type key and ``tensor``
+  type values.
 
 EdgeBatch
 ---------