Required docs updates

Summary

The docs require a bit of touching up before the release.

Here's a list of work items that require completion before 4.0 release:

New logging is documented. The transition from 3.2 logging to 4.0 logging should also be documented. See #605 for details. (resolved in #674)
VoiceNext updates from #566 should be included. (resolved in 50eb633c)
Intents should be mentioned in the library somewhere, as well as in the upgrade guide. (resolved in #673)

If anything else comes to mind, do let me know.