MSFT Visio: How to make "documentation" field show in in a UML diagram?
Solution 1:
You've not specified a version of Visio, but here's what I've got for Visio 2003.
Once you've created an object on the page, right-click and select Shape Display Options..., ensure Properties is ticked and return to the page.
This doesn't seem to work for some object types (because Properties cannot be enabled) and I personally don't like the way it is displayed, but hopefully that might help.
The alternative is to just add Note objects to your diagram instead, but then the text is stored in the diagram itself and not the UML model.