Added to database transaction description.

I've added to the [database transaction](http://laravel.com/docs/database#database-transactions) description in order to note two important things:

1. The behavior of transactions when exceptions are thrown (giving developers the knowledge of how to cause a rollback in their code, as it is not specified)
2. Added information about the availability of the `beginTransaction`, `commit` and `rollback` methods, which developers may have use for when if need to both catch an exception AND also be able to rollback/commit a transaction.