Update postcss plugin docs?

The docs suggest I should use doiuse as a postcss plugin like this?

```
postcss(doiuse({onFeatureUsage:function(usageInfo){
alert(usageInfo.message);
}})).process(styleSheetContent)
```

But, I had to use it this way:

```
postcss([doiuse]).process(styleSheetContent).then(function(result) {
alert(result);
});
```

?