Clarifying Bundle Best Practices is for *reusable* bundles

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs? | No
| Applies to    | 2.3+
| Fixed tickets | 

Hi guys!

I wanted to be a little bit more clear that the bundle best practices doc is all about structuring your reusable bundles, not necessarily your application bundles.

I realize I broke from the Cookbook-standard of "How to..." title, but this title sounded the best to me. I'm also not worried about breaking the anchor on the top title, since it's already the top of the page.

Thanks!
typo: ``used``