Yes there is a documentation. Did a little research

You can now add beneficiaries to your posts directly on Steemit. This will be adapted to Busy once the new version is live. For information, here is the Commit.

So basically, all you have to do is installing or updating your version of SteemPlus, make sure you have the v1.7 and you are good to post!

Let's say you want to post an article about how awesome it was to meet other Steemians at the SteemFest and you want to let them share part of the reward, it has never been easier!

You can get the detailed explanation @steemplus post 6 months ago. Here is the link

https://steemit.com/utopian-io/@steem-plus/steemplus-1-7-share-your-rewards-with-your-friends-beneficiaries-ideal-for-the-steemfest