Auto generated docs for some ops are lack of necessary description 

1. Like ```elementwise_div``` op: http://paddlepaddle.org/docs/develop/documentation/en/api/v2/fluid/layers.html#elementwise-div
```paddle.v2.fluid.layers.elementwise_div(**kwargs)```
```**kwargs``` should be replaced by actual arguments with well explanation.

2. And some ops are lack of docs