Yes, it's called 'custom properties' and they're generated into the xml doc comments on an entity class. (or field/navigator if you add them to those). You can also access these in your custom templates if you want to.
The easiest to add them is by opening the general entity editor (Project -> General Entity Editor) then click an entity in the project explorer and go to the Code generation info tab, then to the Custom Properties tab and click New to add a name-value pair. To add a description to another entity, click another entity in the project explorer, the tab selection stays the same so you can just click 'New...' and add a name-value pair there.
Additionally, a modelview has a documentation pane too, which might be a good place to give context to a set of related entities. It's docked to the left side of the model view viewer.
You're going to use the description in a given context somewhere, e.g. custom templates? (so we understand the use case and can e.g. make the editing of these easier (as it's currently a little cumbersome) and perhaps add it to our system )
