On a site similar to this one I answered a question and the OP made a comment which prompted me to complete my answer in an edit. I called it "an example" but I originally wanted to call it "supplement" or "complement". So, I wonder: is either of these titles correct in this context? Both? Both with a different nuance of meaning?
Supplement would refer to extra/additional information in this context. Complement refers to an item that completes or goes well with another item.
Personally, I'd be inclined to refer to an 'example' edit as a supplement to your original post. If the addition of an example makes your answer more complete though, you could certainly refer to it as a complement.
I'm not sure I'd use either in this case but complement would be my first choice of the two (given the context that what you provided was a completion.) Further addition/s could then be labeled supplement when / if you find an update is required due to a previous flaw or whatnot.
To be honest though, given the nature of sites like this, maybe I'd go for addendum. Reason being that I find requests on such sites to be like technical specifications, inasmuch as neither are often frozen in their requirements - 'completion' becomes a rarity, and additions seemingly perpetual.
AJ01 correctly defines the different meanings of the two words, but I think the main difference between them is that you shouldn't use complement at all unless you're very sure why you want it.
Complement is quite rare outside specialised contexts (music, geometry, computing, etc.). It doesn't really mean "goes together well with" - people just assume that because one of the few 'correct' common usages is, say, Mint sauce complements roast lamb. Where it means "makes complete", not just "is good with".
If only to avoid confusion with the totally unconnected compliment, I suggest you steer clear of this word. Supplement is perfectly valid for your example.
A Complement usually makes the word that it goes with complete or perfect. A Supplement, however, only helps that word. It is purely supplementary. Therefore it depends how you would like to make yourself mean.
In your question, either word would have been correct.
So if your answer makes your question perfected, then good. However, if it helps this your answer but doesn't necessarily perfect it, then put supplement.
A complement is always the response to incompleteness that provides the required essential part. For example: Soul is the complement for a human body to become a human being (without soul the body is not human being.
A supplement is always the response to extra needs or demand. For example: Spare Tyre is the supplement for any vehicle (without the spare tyre it’s possible to go anywhere but it’s essential to have one)