An interesting idea.
I used documentation for entities and attributes where the naming of each element doesn't completely describe an element. I also use this documentation for associations. However, it would be good to have a way to view all of that documentation at once - maybe in this window:
Or possibly all documentation for the whole domain model of a module. Maybe you can suggest this in the ideas tab of the forum - I would vote for it!
Best practice in my opinion is to always use the 'Documentation' area in an entity (and in a attribute) to give a short explanation of the necessity, use and introduction of the entity. For the person who creates it, this may often not seem necessary ("it's self-explanatory right?"), but for people new to a project it is very helpful. Plus, it's a small effort.