I'm sure I'm not alone in thinking that Swift had made an excellent choice in going with reStructuredText for doc comments. I know Markdown is more ubiquitous, but it's also somewhat ill-defined and isn't particularly well-suited to long-form text. The use of reStructuredText promised to have a much more standardized way of writing documentation that could be very expressive and support extensions in a well-defined way (e.g. so documentation tools could define extensions you could use in your code).
The only real concrete benefit to using Markdown is unifying it with Playground rich comments, but it seems to me that Playground rich comments should have just been using reStructuredText to begin with (or perhaps could have had a toggle in the document inspector for comment style, if there's a real need to support Markdown here, although I don't know what that need would be).