I've been working remotely for more than 5 years straight. In my experience working physically close to persons in your team makes sharing information much easier and faster. In fact, it is usual that companies offering remote work to ask new joiners to show for a short amount of time to the main HQ. Also, it helps a lot when the team joins from time to time in the same physical location. In addition to sharing ideas it fosters team building, and the remote communication afterwards is much fluent.
Said that, the most effective way of communication for sharing knowledge is videoconference. As a mentor, you can see the face of the other person and know if they understand or need a bit more time/different explanation. That's something difficult to achieve just via phone call, chat or plain documentation. It is not a substitute of them, but a good improvement.
As of this day, many persons prefer voice calls over videoconference, which looses many of the benefits forementioned. Many of this persons tend to be in the 'old' range. The new generations seem to be more used to this. So I wonder if this is going to be just another barrier.