[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
2.3 only IMO. We don't merge bug fixes in 2.0 anymore (even security patches btw). Fixing the XSD is not a security patch (if nobody complained until today, it is because very few users are using XML for the bundle config)