docs: ngSanitize is out ng core at AngularJS 1.2.21

When using ngBindHTML directive, ngSanitize module must be added as a dependency.

Not doing so results in the following error:

```
Error: [$sce:unsafe] http://errors.angularjs.org/1.2.21/$sce/unsafe
    at Error (native)
    at http://somesite/js/angular.min.js?n9wkqz:6:450
    at e (http://somesite/js/angular.min.js?n9wkqz:117:34)
    at getTrusted (http://somesite/js/angular.min.js?n9wkqz:118:327)
    at Object.e.(anonymous function) [as getTrustedHtml] (http://somesite/js/angular.min.js?n9wkqz:120:71)
    at Object.fn (http://somesite/js/angular.min.js?n9wkqz:192:234)
    at k.$digest (http://somesite/js/angular.min.js?n9wkqz:109:213)
    at k.$apply (http://somesite/js/angular.min.js?n9wkqz:112:173)
    at h (http://somesite/js/angular.min.js?n9wkqz:72:300)
    at w (http://somesite/js/angular.min.js?n9wkqz:77:288) 
```

You can see that the API reference for ngBindHTML includes the module below:

![selection_004](https://cloud.githubusercontent.com/assets/108130/3837185/9cdf6e08-1de6-11e4-93fa-a4a99b919e8c.png)