I have discovered an answered question with a wrong answer. I've found it initially googling by myself to an answer to it, and I've fixed it. Also I've tried to do what an answer suggests - and it is NOT working. I've posted my working answer as a comment - how can I add my correct answer, so other googling people won't be misguided by wrong answer accepted?
You have an account with 101 rep on the main site. The "protected" banner says:
This question is protected to prevent "thanks!", "me too!", or spam answers by new users. To answer it, you must have earned at least 10 reputation on this site.
However, there is a subtlety here in the wording, "earned". The 100 bonus you receive for being a member elsewhere on Stack Exchange doesn't count in this instance:
Who can answer a protected question?
Users with 10 or more reputation can answer a protected question. However, the +100 account association bonus is ignored for this check, so you must have earned 10 or more reputation on that specific site to answer a protected question.
Since you don't have that, the "protected" restriction applies to you. You then have a couple of options:
Be satisfied with the comment you left; in my opinion in this case that's sufficient. The comment is clear and at the top (except for one brief thing above it). Anyone who reads the answer will see it. The person who wrote the answer is not active here anymore, but s/he is active on S.O. so would have received notification of your comment by now.
Post your own question and provide your own answer -- if you do this, be sure to differentiate it somehow from this one. E.g., you've implied that the documentation itself in this configuration file is wrong. Is it still the same on Jessie? I could not find this file in that location on a current Jessie lite image, so I can't say, but it seems something has changed. I checked on another site where I have the same 101 rep and there is the "Answer your own question" tickbox at the bottom of the "Ask Question" page, these are encouraged (presuming you make good use of the mechanism).
Since this is a gray area cross-over topic with Unix & Linux, which has a much larger user base and available expertise on this kind of issue, you could ask about it there. Hopefully, the issue would then be definitively settled, and you could add another comment with a link to that question, which would be more likely to attract the OP's attention and that of the general reader (particularly someone who's looking for a solution to a problem).
Note that you cannot edit other people's posts arbitrarily without approval until you have 2000 rep. However, editing a wrong answer to say "EDIT: This is wrong", or (worse) to substantially change the content because you think it is wrong is absolutely inappropriate regardless of reputation. You use comments or you add your own answer, in which you are free to reference other existing answers you believe are wrong.
You can also use flags if you believe something deserves moderator attention, and in extreme cases, feel free to access the chat, where currently there is pretty much always one mod or another lurking around whom you can ping for attention.