minor #15421 [Doctrine] add missing mapping type (PHP Attributes) (xthiago)

This PR was merged into the 5.3 branch.

Discussion
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[Doctrine] add missing mapping type (PHP Attributes)

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Support for [Doctrine ORM 2.9 AttributeDrive was added a time ago](doctrine/DoctrineBundle#1322), but the docs does not mention that possibility yet.

```yaml
doctrine:
    orm:
        mappings:
            Your\Namespace:
                type: attribute # <-- HERE
                dir: '%kernel.project_dir%/src/Your/Namespace'
                prefix: 'Your\Namespace'
                alias: YN
```

Commits
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8069208 add missing mapping type (PHP Attributes)

Author: javiereguiluz

reference/configuration/doctrine.rst

@@ -265,7 +265,7 @@ you can control. The following configuration options exist for a mapping:
 ``type``
 ........
 
-One of ``annotation`` (the default value), ``xml``, ``yml``, ``php`` or
+One of ``annotation`` (the default value), ``attribute``, ``xml``, ``yml``, ``php`` or
 ``staticphp``. This specifies which type of metadata type your mapping uses.
 
 ``dir``