Ajax: preventing xss on untrusted URLs

Per https://github.com/jquery/jquery/issues/2175, I recommend we add a note on the ajax page about retrieving scripts from an untrusted domain. Automatic JS execution and JSONP type inference can be prevented with options. All that is required is to set the dataType...

``` js
$.get({
  url: "//untrusted.com/script.js",
  dataType: "text"
});
```

http://jsbin.com/rafima/1/edit?js,console
