[Doc] Dgl glance tutorial improvement (#197)

* update dgl at a glance

* update title

* Update 1_first.py

tutorials/1_first.py

 """
 .. currentmodule:: dgl
 
-DGL at a glance
+DGL at a Glance
 =========================
 
 **Author**: `Minjie Wang <https://jermainewang.github.io/>`_, Quan Gan, `Jake
@@ -112,33 +112,40 @@ plt.show()
 def super_useful_comp(g):
 
     def send_source(edges):
+        # 1. pass the source node feature 'x' weighted by edge feature 'w'
         return {'msg': edges.src['x'] * edges.data['w']}
 
     def simple_reduce(nodes):
+        # 2. perform reduction on received messages and update feature 'x'
         msgs = nodes.mailbox['msg']
         return {'x': msgs.sum(1) + nodes.data['x']}
 
-    def readout(g):
-        return th.sum(g.ndata['x'], dim=0)
-
     g.register_message_func(send_source)
     g.register_reduce_func(simple_reduce)
 
     g.send(g.edges())
     g.recv(g.nodes())
 
-    return readout(g)
+def readout(g):
+    # 3. read the aggregated node feature 'x' on graph
+    return th.sum(g.ndata['x'], dim=0)
 
 ###############################################################################
 # See the python wrapper:
 
 g_boring = a_boring_graph()
-graph_sum = super_useful_comp(g_boring)
-print("graph sum is: ", graph_sum)
+graph_sum = readout(g_boring)
+print("graph sum before send() and recv() is: ", graph_sum)
+super_useful_comp(g_boring)
+graph_sum = readout(g_boring)
+print("graph sum after send() and recv() is: ", graph_sum)
 
 g_better = an_interesting_graph()
-graph_sum = super_useful_comp(g_better)
-print("graph sum is: ", graph_sum)
+graph_sum = readout(g_better)
+print("graph sum before send() and recv() is: ", graph_sum)
+super_useful_comp(g_better)
+graph_sum = readout(g_better)
+print("graph sum after send() and recv() is: ", graph_sum)
 
 ###############################################################################
 # Next steps