[WIP] Documenting all options of the FrameworkBundle

Let's give one of the most forgotten dark places of the documentation some love :)

It's quite a difficult job, since I have to document lots of things I'm normally not very interested in (and thus do not know much about). So I hope the core team can help me verify the documentation in this PR. (cc @stof @jakzal)

| Q   | A
| --- | ---
| Doc fix? | yes
| New docs? | yes
| Applies to | 2.3+
| Fixed tickets | not sure
Really? From [what I see](http://symfony.com/doc/current/contributing/documentation/standards.html#example), we only give examples for the first three levels. Nonetheless, if it's working I'm fine with that. I don't think we should show more than three levels in the standards since you usually shouldn't nest the sections that deep.