Clojure uses its metadata (attached to values, though, not declaration) to put documentation strings there.

That's quite doable with D:

@(doc("This function does.... "))

auto foo()

{ ... }

I'm also interested in tagging data:

@(Sorted!(withThisFun)) someRange ...

@(Ranged(0.0, 1.0)) someRange ...

or:

@Validated SQLQuery query;