In PSI,
You can checkout a Project and also set a session description by using this method
ProjectService.CheckOutProject(projectUid, sessionId, sessionDescription);
In CSOM,
The closest available method is
DraftProject prjDraft = prj.CheckOut()
How do i set Session Description while checking-out the project using CSOM?
PSI is not supported in 2016
Hammad Arif EPM Advice Blog