Add comment to `OpTestMeta` (#2968)

python/paddle/v2/framework/tests/op_test_util.py

@@ -5,6 +5,18 @@ import paddle.v2.framework.create_op_creation_methods as creation
 
 
 class OpTestMeta(type):
+    """
+    Operator Test ClassMeta.
+    
+    It injects `test_all` method into user's OperatorTest class, to make Python 
+    unittest module run that method.
+    
+    The `test_all` read what value is stored in `self`. It use self's values to
+    create and run a operator, and check whether that op is OK or not.
+    
+    See `test_add_two_op` for example usage.
+    """
+
     def __new__(cls, name, bases, attrs):
         obj = super(OpTestMeta, cls).__new__(cls, name, bases, attrs)