What is the colon (:) thing about?

E.g.

    /*:
       Some comment
     */

Is there a legitimate reason for the non-C addition atop a C-syntax?

It seems to be a convention that means "this comment contains Markdown and should be rendered in the generated documentation" as opposed to commenting out unused code or whatever.

it's to mark the comments in these blocks to be extracted for formatted documentation generation rather than conventional code comments

Weirdly, it doesn’t seem to be DocC (https://developer.apple.com/documentation/docc)