Markdown editing help is a big help for anyone who is at least a little acquainted with computer use. Using editing interface is pretty obvious for people like us. On Stack Overflow and other computer-related sites, this level of computer familiarity is a necessity, but there are other sites completely unrelated to programming and computer use, where new users who don't understand how the formatting interface works come in relevant numbers. We should help them, not to require them to understand everything just because founders of the site were programmers. Editing help as it is now is not very helpful to such people and there's no low-threshold help (or at least I don't know about it). The result is that some users don't learn how to use formatting for months and they produce not-so-well formatted posts in the meantime. Well, we experienced users try to help them, but it's tiring to teach someone the same basic skill every month or so.
EDIT: the "editing basics" help would be better placed as an extra topic. First I thought it might be part of some of the existing editing topics, but there it would annoy us who consider the interface use obvious.
EDIT2: this question shows that there are people who need extra help with formatting of code blocks. Well, the question was asked when there wasn't so good support as is now, but from people who ask on Stackoverflow you can expect much higher level of computer literacy than for enthusiasts or experts at say cooking who might have problems to learn how to use web browser and Google. If they don't understand it in the first place (well, all the help @Bart pointed to anyone with at least mediocre computer operation skill must understand it if they try, but some users lack one or both from basic computer literacy and interrest for the quality of proper formatting), I don't know anything like this, just for non-programming sites. It doesn't have to be a help topic necessarily, just something I can find easily and send new users to if they seem not to have understood all the help which is there for beginners.