[Templating] Added a sentence that explains what a Template Helper is

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | n/a

I found a SecurityHelper in the SecurityBundle. I was unsure what this helper was or what it was supposed to do. After some googling I've landed on the templating component documation. This is correct as the `HelperInterface` can be found in [the component](http://symfony.com/doc/current/components/templating/introduction.html#helpers). However, it doesn't explain what a helper is.

After some searching, [the cookbook *does* explain what a helper is](http://symfony.com/doc/current/cookbook/templating/PHP.html#using-template-helpers). I've pretty much copied over the sentence from the cookbook to the component. If you have a better proposal, I can add that instead.