Welcome back to the fold! 
Sync is covered in the Help for each application:
- For DEVONthink, sync is covered in the Help > Documentation > In & Out > Sync, Preferences > Sync, and Troubleshooting > Sync Issues.
- For DEVONthink To Go, sync is covered in the ? > Help > Sync your databases section.
Additionally, there is a forum post on syncing: Sync Types Explained