It's been there since 3.0 was released ;)
I'd recommend expanding any help pane (click the icon next to close) as you're right, it's hard to see everything in that little window.
Our plan with documentation at the moment is to provide some guides for the areas that can't be covered inline (i.e. getting started, skinning) that take a more "examples" approach - the inline ACP documentation will continue however to be the main documentation source.
The problem with that massive pdf we had for 2.3 was not only was it difficult to update, while it seemed very thorough, the whole thing was pretty much "To add a forum, click 'Add Forum'" - which isn't really very helpful. We much prefer the inline help.
The habit one should be getting into is using the ACP search to find the page you need, and then use the inline help if you need it. Once you get used to it, I think you'll find this easier than opening the pdf, searching that, and then going back and forth from the pdf to your acp to do what you want to do.