DIC Service Definition explanation

This is not meant to be merged, it's just inspiration. Most of the time, the DIC Compiler exceptions are really confusing. This helps a little 

![screen](http://dl.dropbox.com/u/32120652/nette-servicecreationexception2.png)

It would be nice, to have something like this supported by default. But it's way to complex for me to figure out where to hack.