Why am I not able to select a model for documentation?

You cannot select a model if this model does not have any owner assigned to it in requirements. Go onto Requirements > All Discipline Models to select the owner for your model. 

If you are not able to see the owner as a seperate column, you need change the view, using the Requirements View, just click on Owner and save your settings. Now, you should be able to define an owner for your model and select this model for documentation.