Update Future docs for TinkerPop 4

[xiazcy]

The Future docs for TP4 has been outdated since we've released 3.7.x. This is to update the Future section to align with the TP4 features that has been in discussion or have been brought up at various points.

I based this on master since it is around TP4, but can rebase to 3.7.x if we want this published into the latest docs.

Note I added some formatting to the features doc, so opening the file in rendered view might be easier to review.