docs: Describe how to update N Preview test devices on N release

Noted that devices that are signed up for OTA updates will
automatically update to release version; devices that you flash
manually (and don't subsequently sign up for the beta program) will
need to be flashed with the production version.

See first comment for doc stage location.

Change-Id: I6755c837f80a459776880422f6d84af1d0b75f34