refine data_feeder doc

test=release/1.2

python/paddle/fluid/data_feeder.py

@@ -258,10 +258,13 @@ class DataFeeder(object):
         multiple mini-batches. Each mini-batch will be feed on each device.
 
         Args:
-            reader(fun): the input data.
-            multi_devices(bool): the number of places. Default None.
-            num_places(int): the number of places. Default None.
-            drop_last(bool): the number of places. Default None.
+            reader(function): the reader is the function which can generate data.
+            multi_devices(bool): whether to use multiple devices or not.
+            num_places(int): if the multi_devices is True, you can specify the number
+                of GPU to use, if 'num_places' is None, the function will use all the
+                GPU of the current machine. Default None.
+            drop_last(bool): whether to drop the last batch if the
+                size of the last batch is less than batch_size. Default True.
 
         Returns:
             dict: the result of conversion.