Document properly the semantics of MigrationInfo
Document properly the semantics of MigrationInfo

- lastMigrated can be null if no change was ever migrated
- lastAvailableToMigrate can be null if there are no new changes to
migrate

PiperOrigin-RevId: 147786871
Change-Id: I1f6cb9958c5966b757753f2c12af0ac2d5c2edcd

google/copybara,google/copybara,google/copybara