Kurt Buhler shares some thoughts:

In a Power BI semantic model you can set freeform text fields for each object (like tables, columns, and measures) to describe what they do, how to use them, or other information. These descriptions are a convenient and structured way to document each object for developers. They’re also helpful for users, since the descriptions (unlike DAX expressions) show when you hover on the object in Power BI Desktop:

Click through to see what Kurt recommends in terms of items that should go into a description, as well as things that do not belong there. Kurt also has some strong ideas around AI-generated descriptions and what makes a description relatively more useful for a person versus a language model.