I don't think a concise symbol is needed. A Markdown area, when activated, should have some sort of sidebar hint to give basic examples of the syntax, and that

I don't think a concise symbol is needed. A Markdown area, when activated, should have some sort of sidebar hint to give basic examples of the syntax, and that takes enough space that Markdown can be used as a word. Recognition might be speedier for the spelled-out word, too. Before the user decides to input something, advertising Markdown support isn't very important.