Improve colorscale attribute description

More info here: http://stackoverflow.com/a/34752259/4068492

To be done [here](https://github.com/plotly/plotly.js/blob/master/src/components/colorscale/attributes.js#L37) and [here](https://github.com/plotly/plotly.js/blob/master/src/traces/scatter/attributes.js#L249-L250).
