We've always been asked to do the basics, like clean up the poop if your horse goes in the aisle, or wash rack, and if you do your own blanketing and unblanketing, that you put the blanket up nicely on the stall door, or pipe fencing. Barn maintenance on the other hand is what the BO does, or what he/she pays other workers for. The people who clean the stalls also normally do the things like that, raking up around the hay barn, fixing things that need fixing etc. Stuff like cleaning cobwebs, even cleaning some stalls, helping with summer camp, are what you do if you are looking to work off a little of your board, but paying for board, full care or not, you should not be asked to do stuff like that. I hate confrontations as well, but something as simple as "I didn't know that was in the boarding contract, so I looked it up, and it's not, is this something new", to start, or "I saw the note on my horse's stall, and doing stall/barn maintenance is not in my contract, it states that the BO takes care of that" just make sure that it is actually stated in the contract first obviously. And I would definitely not just do it, and not say anything, and hope that's the only thing that will come up.
I was working at a place where my job description was CLEARLY defined, and one day the BO asked if I could help out with something, just for that day, so of course being helpful, and having some time before my second job, I said sure, and helped, but for some reason the BO then decided without saying anything, that it was just one more thing I would do around the barn. After a week of it apparently not getting done, he came up and got upset at me for not doing my job. There were several things that he just decided to add to my job description, so I ended up having to sit down with him, and get into a long drawn out conversation about what I had really agreed to do, and that I had a second job etc. etc. Needless to say, I ended up quitting after a few months, because he kept trying to pull one over on me. So definitely nicely bring it up because it isn't your job.