migrations: Update documentation

Created by: efritz

Update documentation:

• dirty database docs should improve example shown (should explicitly call out add-log ....)
• add docs for locating migrator logs in k8s (see below)

Problem: Customers may have difficulty finding logs from the migrator during a failed migration. Running kubectl logs showed Error from server (BadRequest): container "frontend" in pod "sourcegraph-frontend-69f4b68d75-w98lx" is waiting to start: PodInitializing. You need to specify kubectl logs -c migrator in order to get the actual logs.

See https://github.com/sourcegraph/sourcegraph/issues/30362#issuecomment-1042335944.