No, there's no tool for documenting .xib files unless you consider the file itself as part of your documentation. (You did say "self documenting" after all.) .xibs are stored as XML, though, so if you really want to extract the data, it's there to be had.
The actions you create in IB appear in your source code. You could add comments there describing the object that sends the action, but that would largely miss the point of using a .xib (and actions) in the first place, which is to decouple the UI elements from the controller.
If the point of the documentation is to make your project easier for other developers to understand, you might want to get some more experience with Objective-C before you go to a lot of trouble creating a .xib documenting tool. Given a .xib and the relevant header files, any competent Obj-C programmer should be able to tell easily which controls are connected to which actions, or to debug problems in that area. People have been working with .xib and .nib files for 20+ years -- the lack of a separate documentation tool may be a good indication that such a thing may not be as useful as it seems at first.
