Revises section that advises an insecure method for configuring migrations

- also moves step-by-step instruction to Cloud Admin Guide rather than config ref

Closes-bug: 1287194

Change-Id: I21694c7b2c5c6cf6bcdbbb7b4ee9f6b8597424d7
Merge "Revises section that advises an insecure method for configuring migrations"