we need to update the doc of scatter_nd and gather_nd

[scatter_nd](http://mxnet.incubator.apache.org/api/python/symbol/symbol.html#mxnet.symbol.scatter_nd) and [gather_nd](http://mxnet.incubator.apache.org/api/python/symbol/symbol.html#mxnet.symbol.gather_nd) in MXNet are similar to the ones in TensorFlow. But the current documentation is difficult to understand. We need to update the document.