[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
I'm in favor of leaving the YAML dump as well, but I removed it because of this comment by @weaverryan years ago:

> The big YAML tree was almost more of a temporary fix until we actually found more time to document the individual options. In a perfect world, I think we might not have any big tree, but rather the keys themselves are all documented, and an example of how you configure that one option is provided in YAML, PHP and XML when describing that one option.
> https://github.com/symfony/symfony-docs/pull/2076#issuecomment-12758728