[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
The schema is not retrieved by requesting the url. Symfony's Xml parser replaces the configured extension namespace with the file path returned from `getXsdValidationBasePath()`.

The FrameworkExtension looks like:
````php
class FrameworkExtension // ...
{
    // ...
    /** 
     * Returns the base path for the XSD files. 
     * 
     * @return string The XSD base path 
     */ 
    public function getXsdValidationBasePath() 
    { 
        return __DIR__.'/../Resources/config/schema'; 
    }. 

    public function getNamespace() 
    { 
        return 'http://symfony.com/schema/dic/symfony'; 
    } 
}
````

So `http://symfony.com/schema/dic/symfony/symfony-1.0.xsd` becomes `path/to/FrameworkBundle/Resources/config/schema/symfony-1.0.xsd` which is the right schema in the right version