A good reason for downvoting a question is a clear lack of research, asking more or less somebody else to solve your problem without having done any effort. Now, let's consider the particular case of a question posted together with a self-answer. I think it is a reasonable approach to put the research part basically into the answer. Somebody can disagree, that's not my point.
However what can happen is that a user might see the question and quickly downvote it for lack of research, not noticing that it had been submitted already with an answer. I am not able to point to any demonstrated case, I just want to point out that it can theoretically happen.
So my feature request is: Let's automatically mark question posted together with an answer with a clear information box like:
This question was intended for documentation purpose. The author has already provided an answer to the problem. Read about self-answered questions. You are welcome to submit alternative answers!
To avoid ambiguity, I am not speaking about a question posted and only later on self-answered. Also, I am not saying that an immediately self-answered question shouldn't be downvoted in any case, only that the downvoter should take an informed decision. I also want to mention the possibility that if you have never experienced this it might also been that you have been shielded by a good reputation score, which induced the possible downvoter to take a closer look to a "weird" situation (Why is this highly experienced user asking questions in this way?)
Not to hide (completely :-) ) anything, the question is inspired by another discussion which I prefer not to link here (if you are really curious you can try to find it, but I think it won't help to judge this feature request).
Edit: I now see that this idea has been previously brought up. In that case the proposed banner was:
This is a self-answered question, a behavior encouraged by stackoverflow. Vote on this answer as you would any other. You may offer an alternative to the original poster's answer.
The top answer suggested:
You might try including something like this at the top of your self-answers:
I encountered this problem earlier and managed to find the answer myself, so I thought I'd post it here in case anyone else is searching for the solution to this issue.