From time to time, people have accidentally stumbled into single-document mode and have been really confused about what happened and how to exit it (for example, see#8286). I suggest we:

1. Remove the default keyboard shortcut. It seems that people mostly stumble into it by accidentally pressing the default keyboard shortcut. Perhaps we rely on a menu item by default, and document how to set up a shortcut if desired.
2. Add a status bar item when in single-document mode. Clicking on the item could exit single-document mode. The purpose here is to make it more obvious when you've stumbled into the mode and give something you can click on to get out of it.