I just asked a user to separate his two (otherwise good) questions (real-world use) into two separate questions (stack-exchange use). That is, he was doing a single task, and was wondering about two different aspects of it.
This rule can seem arbitrary to users, for whom it is natural to ask for all the help they need with a single task in a single "package". So I am always careful to explain some of the reasoning behind it when I do it. This explaining has some downsides: 1) it takes time, 2) it doesn't look as "official" when it is just a comment, 3) users couldn't have known about it even if they wanted.
My suggestion is to expand the text in https://cooking.stackexchange.com/help/how-to-ask with a heading of "only one problem per question" and explain a bit of the background. This is a low-friction solution (adding some more relevant text to an existing help page), can be linked when needed, and addresses the other concerns too.