How do I add text comments to a blender file? If I select "Text Editor" in the top left corner dropdown, I get some buttons and other control elements that allow add texts, highlight syntax, run scripts.. but where is the text edit area itself? I'd like to actually add some text, not only create a text file/node/whatever it is called. Even the manual leaves me with no idea of how to actually write text notes.. Am I using a wrong tool? How to add some text abstract to my file?
In order to access the text editor you would have to drag the window down. Try to change to the text editor from the 3d-view. 

As the Info Area is basically just the header, when you switch to text editor you will just see the editor header. Try pulling the header down from the interface to the 3d-view and you will see the text input space appear. (note recommended).

Instead maybe switch the timeline area (usually bottom left (clock item)) to text editor and put something in there. 

I recommend reading that.

https://www.blender.org/manual/interface/introduction.html