@chez yes, I think I understand what you mean. I think it might be ok to continue without a warning even if the post is old.
Local guide to Markdwon and post editing within the post editor
I just installed the Colors plugin on my system and came to the realization that visitors to the site will not know how to add colors to texts and many might not even know how to use Markdown.
Now, I know that there is a link to Markdown at the bottom of the post editor, but I'll like to add some post editing/creation instructions specific to the site that are not part of Markdown. An example would be the feature that the Colors plugin brings to the table.
So here's the feature request, if it's doable: A Posting Guide or Help tab after the Preview tab that's essentially a static page that contains instructions on the basics of Markdown and optional features like adding colored text. It will, of course, be editable and created by the admin or publisher.
I agree (to an extent), but I'll also point out that other forum softwares don't provide a guide as to how to write/format BBCode. That's not to say that we shouldn't just because others don't, just something to think about.
The other thing is to take into account that markdown is just one parser in use. We could put help instructions into the composer, but if somebody switches their parser to BBCode (there is one for NodeBB), then the markdown help is not correct at all.