Seen as the feature is enabled again, I thought i'd breathe some life into this.
Press "Room Travel" under the "Control" heading. Just above the room map of your ship/vehicle/facil/entity you will see two buttons "Room Map" and "Description". Click Description, then Edit (just below the text box). Et voila, you now have two input fields to decide the name, and description of your room.
When you're done, press "save room description" then click back onto the Room Map