Document ALREADY-LOADED-SYSTEMS.
There was an @findex
entry for this function, but no documentation. Added that.
This likely is not where the doc for this function should go.
I will probably just merge this, but any reviewers are welcome to suggest that I move this documentation to a different location.
Assigning this to you @frideau so that you see it. Feel free to ignore if you don't have time; it's a micro-change.