I started documenting all the things missing from that site, but I got bored of that and stopped. The site is missing some newer things in n2-n7.
As far as my documentation method, I opened the Options, clicked OK, commit the mirc.ini in git. Change a setting and compared the difference via git diff. I'd document what it changed, undo the change in Options, change another thing, document that, and repeat...
Warning: These might not be accurate, please verify yourself.
n2,37=Automatically log status: (0)->Off; (1)->On
n6,36=Nicklist position (not sure?)
n6,37=Enable SendMessage Server
n7,38=Use Control Key enable mark/copy
n8,01=Position of switchbar: 0 = bottom ; 1 = top ; 2 = left ; 3 = right
n8,02=Position of treeview : 0 = bottom ; 1 = top ; 2 = left ; 3 = right
n8,03=Treeview Enabled/Disabled: 0 = Disabled ; 1 = Enabled
n8,04=Width of treeview in pixels. Default 168, "x < 24 || x > 500"
n8,06=Include desktop windows
n8,09=View->Mode: - Normal or Compact
n8,13=Lock bars: 0 = unlocked ; 1 = locked
n8,14=Single Click on Tray Icon
n8,15=Flash On: Chat Message
n8,16=Beep On Chat
n8,14 n8,15 / n8,16 were added in 6.21. Just look at the change log and try changing new settings that were added.