I just spent some time figuring out how to create a new 'restriction' on a custom item that I created for a Companion added from a mod. Creating a new restriction is something that I've seen others do so I knew it was possible....
Is it worth creating a new tutorial on the toolset wiki?
or
Is it worth updating the 'create new item' tutorial?
Or is it 100% intuitive to everyone expect me so it isn't worth bothering? 
I'd say create a new tutorial (it can link back to the create a new item tutorial easily enough if that is a prerequisite).
Yes, I would definitely link back to the create new item tutorials; no need to repeat what is already there.
I'll have a bit of time (I hope) in a few weeks to create new 'restricted' items. I'll try to write the tutorial at the same time so that I can document as I create the new restrictions. Of course, I know what I did this time, but that doesn't mean that I wouldn't forget a step or reverse a couple of steps if I tried to write a tutorial from memory! 
Do góry