docs: explain which SQLMesh version to use with rollback

docs/guides/migrations.md

@@ -36,3 +36,26 @@ Migrations should ideally run when no one will be running plan/apply.
 Migrations should not be run in parallel. 
 Due to these constraints, it is better for a person responsible for managing SQLMesh to manually issue migrations. 
 Therefore, it is not recommended to issue migrations from CI/CD pipelines.
+
+## Rolling back a migration
+
+When `sqlmesh migrate` needs to change the project metadata, it first copies each state table to a backup table with a `_backup` suffix, unless `--skip-backup` is passed. The `sqlmesh rollback` command restores the metadata from those backups, returning it to the format used by the SQLMesh version that was installed before the migration. If a migration fails partway through, SQLMesh rolls it back automatically from the same backups.
+
+To undo a migration:
+
+1. Run `sqlmesh rollback` with the SQLMesh version that performed the migration still installed.
+2. Reinstall the SQLMesh version the project used before the upgrade, for example by reverting the version change in your requirements file.
+
+The second step is required. Rolling back does not change the installed version of SQLMesh, and the newer version will refuse to run against the restored metadata until it is migrated again:
+
+```bash
+> sqlmesh plan
+Error: SQLMesh (local) is using version '2' which is ahead of '1' (remote). Please run a migration ('sqlmesh migrate' command).
+```
+
+Keep the following in mind before rolling back:
+
+- Only the most recent migration can be rolled back. Restoring consumes the backup tables, so running `sqlmesh rollback` a second time fails with `There are no prior migrations to roll back to.`
+- The backups are taken at the moment of migration. Any changes made to the project metadata after the migration, such as plans applied with the newer version, are discarded.
+- Like `sqlmesh migrate`, rolling back affects all users of the project and should be issued manually by a single user.
+- Rollback is not possible if the migration was run with `--skip-backup`. It also does nothing useful after an upgrade that required no migration, such as a patch release, because no new backup was taken. In that case the backup tables, if present, come from an earlier migration.

docs/reference/cli.md

@@ -494,6 +494,8 @@ Options:
 
     The `rollback` command affects all SQLMesh users. Contact your SQLMesh administrator before running.
 
+Run `rollback` with the SQLMesh version that performed the migration, then reinstall the previous version. See [Rolling back a migration](../guides/migrations.md#rolling-back-a-migration) for details.
+
 ## run
 
 ```