Markdown doesn't have a comment. There are eleventy bazillion different ways of simulating a comment by abusing things to effectively hide a chunk of text, but they are all very sensitive to which Markdown renderer you are using. The safest bet is a <div> with a style that sets display to none. The <> class does this:

<div class="hidden">
some text that you don't want to appear in the document
</div>

The text is still in the HTML, so don't use this to hide anything sensitive.