This will be difficult for source packages to adopt. Conditional compilation of attributes has never shipped (it is in review right now), so using this in a package would require excluding developers who need to support any currently-released version of Swift, or the imminent 5.7. A comment-based approach would, however, be usable straight away by everybody.
Such a design would require that the markdown processor understand DocC directives, but that seems to be the direction anyway. It would be good to decide just how far we're willing to go beyond standard markdown, so we have a consistent answer about whether these kinds of designs (using directives in doc-comments) are acceptable.
In the future, attributes should be easier to adopt (by source packages as well). But is there an intent for this to go through swift-evolution eventually? The compiler is full of incredibly useful functionality which never goes through evolution and seems to permanently live with a leading underscore. We've thus far not deeply embedded DocC or any documentation engine in to the language itself, and I worry that our hesitation on that is likely to remain, leading to yet another perpetual unstable-but-critical feature in Swift.
But if there is to be an attribute, let's not let it linger - let's get it ready for formal adoption as soon as possible. Let's not give more unstable features a chance to spread and become a defacto part of the language.