So what I would suggest is to have a checkbox with something like "Scribble mode" inside the "Instant save" tab enabling this feature
"Does it save the Geany Scribble window then?" [user who didn't read the docs question]
Whilst I didn't get the "persistent temp files" either and I still don't know what the use-case is beyond persisting unnamed buffers and I don't understand what the reason for saving unnamed buffers, if its unnamed how do I know whats in it to switch to it when they are all named "Untitled"? So I can't suggest anything better than the descriptive "save unnamed buffers".