Improve extender's guide

[ghost]

I think two problems with the current extender's guide are:

1. It does not describe all the levels and how they interact. So, while

it tells what you can create a generator, it's not obvious what generator takes, and when it's called and what it can produce.

2. There's no textual documentation of the important methods of

generators (that one might want to override), or in virtual-target and action class (that one might want to use).

So, the next result is that in parts, extender's guide hints on solutions, but does not explain it in details.