Assetic

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | none

Hi guys!

This builds off of some changes in #1991, which addresses 2 issues:

  a) Warning against using the `@MyBundle` syntax with CSS files (due to the rewrite filter). To my knowledge, this is still an issue with current versions of Symfony. If I'm wrong, please correct me :)

  b) adding a note about the `assetic.bundles` key - not included here, because this PR is against the 2.0 branch.

But moreover, it's basically a re-read/refactor of the first half of the assetic docs. If anyone sees any errors or has any suggestions, please let me know!

This PR is against the 2.0 branch - after it is merged, we'll add a note about `assetic.bundles` on 2.1.

Thanks!