Improve documentation about how to use K3 aspects from pure Java

[dvojtise]

The documentation is still unclear about how to call K3 aspects from pure Java see. https://dev.eclipse.org/mhonarc/lists/gemoc-dev/msg00007.html

For example it should make clear what is the api part of the code generated from xtend (ie. the static methods in the xxxAspect class )

It should also make clear about the use of import static extensions or not (there are some info in some part of the documentation (disambiguisation section), but we should gather/repeat this info in a dedicated section.