feat(migrations): Advertise --reuse-db command for migration test

src/docs/database-migrations.mdx

@@ -88,6 +88,8 @@ class MyMigrationTest(TestMigrations):
 
 To run the test locally, run `pytest` with `--migrations` flag. For example, `pytest -v --migrations tests/getsentry/migrations/test_0XXX_migration_name.py`.
 
+If you would like to speed up the migration tests and do not require rebuilding the databases on each test run, supply `--reuse-db` as an additional option to the test command.
+
 ### Backup Testing
 When you add or change a model, an error message in CI may appear explaining that one or multiple tests "produced an `export.json` backup file that was missing the above models".
 In order to resolve this, there are two steps: