I think it would be worthwhile to have a textarea (multi-line text field) in the entity Designer to display and edit the Javadoc for entities and their fields. This would remind entity designers to include an explanation of the purpose of each entity and of each field, and would provide developers other than the original designer with information about such purpose.
The Javadoc editor for entities could be placed in the top-left pane, while the one for fields would go in the field properties on the right side.
Right now we can include standard Javadoc comments in the code editor, but such documentation is hidden in the designer.
A useful related feature would be a setting in the Jmix plugin to always open entities on the designer tab.