I just use the comments. I make a key and keep it on the event sheet that only has global variables. That way I can flip to that event sheet reference it, and flip back.
(You can move all your global varibles to a seperate sheet by selecting and cutting them and pasting them in the new event sheet.
You will get a warning the events that reference them would be lost, but if you paste them immediately, the events all come back. An event sheet with only global variables, does not have to be included anywhere.)
You can use Shift Enter in the comment to make it a multiple line comment. Here is an image I posted in the past: (mine are green)