Migrations guide: use atomic steps, when possible

Currently, the DB migrations guide says that "you must" use non-blocking
operations (such as CREATE INDEX CONCURRENTLY), always.

But this does not make sense in cases of empty tables and leads to
splitting the work to multiple non-atomic
(with disable_ddl_transaction!) DB migrations.

To follow KISS principle, to have fewer DB migrations steps,
to have them atomic when it's possible and simplify deployment
and troubleshooting, the following exceptions were added
to the doc:

- index creation,
- index dropping,
- defining an FK,
- adding a column with DEFAULT,

doc/development/database_review.md

@@ -91,7 +91,7 @@ and details for a database reviewer:
     concurrent index/foreign key helpers (with transactions disabled)
   - Check consistency with `db/schema.rb` and that migrations are [reversible](migration_style_guide.md#reversibility)
   - Check queries timing (If any): Queries executed in a migration
-    need to fit comfortable within `15s` - preferably much less than that - on GitLab.com.
+    need to fit comfortably within `15s` - preferably much less than that - on GitLab.com.
 - Check [background migrations](background_migrations.md):
   - For data migrations, establish a time estimate for execution
   - They should only be used when migrating data in larger tables.