Cleanup the message factory documentation

[dbu]

the message factory documentation is outdated.

and we should explain how to get those factories, e.g. with a diactoros example. the client doc references http://php-http.readthedocs.org/en/latest/message/message-factory.html , we should explain how to make sure one of the factories actually is installed, list known implementations and so on.

[Nyholm]

we should explain how to make sure one of the factories

We can't make sure a message factory is installed. We would need another virtual package for that.

[dbu]

agreed. but there could be strategically placed class_exists calls to throw a dev friendly exception instead of a fatal error.