I reached out to support and this was the response:
The update to the share button is a new release by development as a way to increase share usage in workfront. This update was given to ~50% of our customers, while the other ~50% have the original sharing option. We have done it in this way to see if adding visibility to the object sharing area will increase sharing on objects or not. The only change that was made for some of our customers was that instead of clicking the three dots on the top of an object, then pressing 'share', they now can just click 'share'. Since ~50% of our customers still have the original share button, and since no changes were made to the actual functionality, no updates to documentation has been made yet. That being said, if this is made a permanent update due to successful testing, then new and updated documentation will be released. I hope this answers any questions you may have with this, but please let me know if you have any follow-ups!