Document steps of switching primary DC for region configurations

These steps are missing from [FDB documentation](https://apple.github.io/foundationdb/configuration.html?highlight=region%20configuration#configuring-regions)

- Acquiring Exclusive lock: primaryCluster
- make sure no data movement or it is completed
- do switch by reconfigure the cluster
- Releasing Exclusive lock: primaryCluster

Start at the linked FDB documentation section on configuring regions and review the existing region-configuration procedure. Add the four listed steps for switching the primary DC, including the lock, data-movement check, reconfiguration, and lock release, then verify the documented sequence against the surrounding instructions.

Written by the indexing model from the issue text.