[WCM] document the namespace alias feature

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | [yes](https://github.com/symfony/symfony/pull/10853)
| Applies to    | 2.6
| Fixed tickets | -

i found no good place to link to. the alias is just used here http://symfony.com/doc/current/book/doctrine.html#fetching-objects-from-the-database and explained in a tip a bit below.