Etsy’s experiment with immutable documentation
I think the part about this that is truly successful, is the use of slack significantly lowers the transaction cost of adding the documentation.  In a wiki, where there is a certain level of quality expected, you might make the decision to just move on with your life once you find the answer you need, as you've already wasted enough time.  With this, however, it's so cheap to leave a breadcrumb it's just polite to do so.