My question is: is there a documentation available where I can find such code / suggestions to change fields with autoactions?

No, you'd need to modify the select options with JavaScript as we've no triggers available to modify field options.

Do you have a documentation for this?

No, we do not have documentation for most internal APIs.

Moreover the whole autoaction system I don't understand enough, some samples I found in the meantime or got from you, but it would be great to have a documentary for using this excellent system more for my cb site.

There's very little documentation for CB Auto Actions. It's not really a plugin we can document beyond explaining the parameters all over again which the parameters do fairly well already. CB Auto Actions doesn't do anything out of the box. It's simply a tool to automate tasks on various triggers. So the best way to understand CB Auto Actions is to understand what triggers you need for what you're trying to accomplish. The trigger tutorial below can help with this as it at least provides available variables for a trigger.

www.joomlapolis.com/documentation/279-community-builder/tutorials/18358-using-cb-triggers

Documentation for CB Auto Actions is pretty much just usecase examples which is either to deal with in forum posts upon request, but we've a few examples below.

www.joomlapolis.com/documentation/291-cb-auto-actions/tutorials

The lack of documentation is purely due to time. I can implement and fix things or I can spend my time writing documentation. There's just not enough time to do both. Hopefully with CB 3.x and some more generic APIs I can reduce the amount of code I need to write per plugin and clear the backlog of documentation, but I'm only 1 person so there's only so much I can do unfortunately and personally will always choose to code over write documentation. Ideally I'd like to figure out a way to nicely embed the documentation directly into the plugins so you don't have to leave your site to get further details about a plugin.