adding note to assetic cache busting

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | 2.5+
| Fixed tickets | n/a

Hi guys!

Just a small note after #4449. I don't know anything about this feature, so I thought we'd at least link to the pull request. But I also wanted people to know that there is another way that's more documented. Really, I don't know why we have 2 ways - or is the cache-busting different between these 2?

Thanks!