Improve @backupStaticAttributes docs

Resolves #226 

Screenies:
#### Annotations (see [original](http://phpunit.de/manual/current/en/appendixes.annotations.html#appendixes.annotations.backupStaticAttributes))
![Annotations](https://cloud.githubusercontent.com/assets/41992/4078814/79733734-2ecf-11e4-8def-a7be5bd3130e.png)

---
#### Global State (see [original](http://phpunit.de/manual/current/en/fixtures.html#fixtures.global-state))
![Global State](https://cloud.githubusercontent.com/assets/41992/4078934/977b080a-2ed0-11e4-9298-1bd9c8206b5b.png)

Sounds good to me. I know many of the other page use second person (like the fixtures page from your PR), but I don't really have a preference.