Argh, yes, that's one line that might make me just walk away from a PR as a new/junior/casual contributor.

You, the reviewer, are an expert in the system. Likely you are the or one of the most expert people in the entire world on this exact thing. You know X exists and why to use it. As you should, because you put it there. You also should know that people who aren't experts (like me) don't know about it, simply because they didn't use it, in this PR, when they should have. Why don't they know it? Probably because you haven't used it consistently in your own code, or it's not documented. This newbie has cobbled this PR together from what sense I can make of this project. Probably 90% is guessed from code I found in there already.

What wouldn't wind me up?

"I think a better way to do this is X. It's better because Y. Or have I missed a specific reason for X?"

Note two things: 1) explanation to a noob of reason Y, which may well be valuable, not only to the noob, but also in the record of the project in general. 2) The indication that the noob might at least have had a logical approach, and they're not an idiot, just a noob.

Afterwards, if this seems like something the noob should have known from the codebase, consider that you, the maintainer, have failed to make it clear.

Of course, if I'm also supposed to be an expert, e.g. a co-maintainer, then it's different. I should know X. Which means either it's a brain fart, or I actually do have a reason. In which case I should have commented on the code, because if another contributor can't tell the intention in the PR, then they can't tell in a year when no one can remember why it went that way.