minor #16792 [Doctrine] Added naming_strategy option to doctrine config reference (naitsirch)

This PR was merged into the 4.4 branch.

Discussion
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[Doctrine] Added naming_strategy option to doctrine config reference

The `namong_strategy` option of the doctrine bundle reference is currenty missing. So I have added it together with its default value.

You can compare it to the complete reference of the DoctrineBundle [here](https://github.com/doctrine/DoctrineBundle/blob/2.6.x/Resources/doc/configuration.rst).

Commits
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99269ac Added naming_strategy option to doctrine config reference

Author: javiereguiluz

reference/configuration/doctrine.rst

@@ -182,6 +182,7 @@ that the ORM resolves to:
             metadata_cache_driver: array
             query_cache_driver: array
             result_cache_driver: array
+            naming_strategy: doctrine.orm.naming_strategy.default
 
 There are lots of other configuration options that you can use to overwrite
 certain classes, but those are for very advanced use-cases only.
@@ -207,6 +208,7 @@ can be placed directly under ``doctrine.orm`` config level.
             class_metadata_factory_name:  Doctrine\ORM\Mapping\ClassMetadataFactory
             default_repository_class:  Doctrine\ORM\EntityRepository
             auto_mapping: false
+            naming_strategy: doctrine.orm.naming_strategy.default
             hydrators:
                 # ...
             mappings: