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 | -

in http://symfony.com/doc/current/reference/configuration/doctrine.html
its auto_mapping in yml, so i go for auto-mapping. also fixed the other
occurence in this page.

automapping|auto mapping|auto-mapping are never used elsewhere in the doc.