Document workstation reprovision/update procedure

[eloquence]

Description

Tails workstations can run at a different version of the SecureDrop codebase than the server, and need to be updated by administrators. However, the exact procedure for updating the workstation varies based on the current state. For future reference, let's consolidate the upgrade docs into a single "Upgrading Workstations" section.

This relates to #3204 as we'll ask admins to ensure their Admin Workstations are up-to-date prior to performing the Xenial update.