Added javadocs for injection and extensions

[jroper]

This documentation could have potentially saved me hours when I was first writing an Arquillian extension - you assume the injection framework is going to work like any other, where you bind components either using annotations or some module API (which is what the loadable extension API looks like with its service registration method), and even though through reverse engineering I'd worked out that components could be provided by InstanceProducer.set(), it never occurred to me that that was an API that I was supposed to use, I thought that was an internal implementation detail that Arquillian would invoke, for example, for each service registered by a loadable extension. So I spent hours scratching my head trying to understand why the services I had registered weren't being injected, and trying to find the code that was meant to register each service with an InstanceProducer (which of course didn't exist).

[bartoszmajsak]

Thank you very much @jroper for improving the docs and apologies this contribution came through the painful path. 🥇