Basically every other programming language has already had something like this for a long time, e.g., Javadoc and Doxygen. Is there a particular reason you'd rather use DocC than these well-established alternatives?
I'd love to use it for Objective-C too. I work on a mixed language SDK and right now use Jazzy which is fine but I think there's a realistic chance DocC will lead to Jazzy being abandoned sooner than we can get rid of all the Objective-C.