That exact link is the real documention about rich edit control (remember, the script editor have one and each channel's editboxes are rich edit control too).
I invite you to scroll down until you can see
Ctrl+Z Undo
Ctrl+Y Redo
Now, see your screenshot ? Why don't you do the same with the script editor and realize control + y is just nicely written next to "redo" ?

Looking for a good help channel about mIRC? Check #mircscripting @