Update HttpFoundation docs to reflect 2.4 changes

[wouterj]

The HttpFoundation component had some changes in 2.4:

• The RequestStack class is added (see added documentation for the new request_stack service #2956 )
• The method Request::getEncodings() is added (see [HttpFoundation] Added Request::getEncodings() method symfony#8185 ). The "Accessing Request Data" section (in the component intro) should document this new method.

[weaverryan]

I'd like to add one, related to this, but really for HttpKernel:

• Document the new FINISH_REQUEST event. This would be done in the HttpKernel component documentation at least. I'm not immediately sure if this is a useful event for developers, but it should be documented.

[wouterj]

@weaverryan I created another issue for that (not sure why I didn't put it here...): #2959 And I know that @xabbuh is already working on documenting that :)

[weaverryan]

The Request::getEncodings change has been merged. So this issue is all set!