Never said it was good, just making an observation that Markdown is most likely to be available to render OOTB in more editors. I don't think Markdown is bad necessarily either. It's "good enough" for simple document.
Documentation isn’t as simple document. There are tons of rich elements missing from the spec specifically for documentation… which is why so many resort to adopting one of the many incompatible forks of Markdown to try to get features that were missing (but are a part of reStructuredText & AsciiDoc for instance). That has a real tradeoff since these forks are not going to be compatible & they aren’t going to be as well defined as the specs for these richer lightweight markup syntax choices.