feature symfony#3946 DQL custom functions on doctrine reference page (healdropper)

This PR was submitted for the 2.5 branch but it was merged into the 2.3 branch instead (closes symfony#3946).

Discussion
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DQL custom functions on doctrine reference page

Custom DQL functions can be configured in two ways, according to what I was told [ here ] (symfony#3939)

It should be clearified in doctrine reference page because It may confuse some people that just read the cookbook  section about [custom DQL functions] (http://symfony.com/doc/current/cookbook/doctrine/custom_dql_functions.html)

Commits
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39957c5 Update doctrine.rst
b4f3151 Update doctrine.rst
9536c17 Update doctrine.rst
044e1eb Update doctrine.rst
1ce3fcc Update doctrine.rst
4a046fa Update doctrine.rst
69efced Shorten syntax mention on doctrine reference
98dee12 Fixed broked link
95f53ae Update doctrine.rst
b5f9ac0 Update doctrine.rst
2541f86 DQL custom functions on doctrine reference page

Author: weaverryan

reference/configuration/doctrine.rst

@@ -411,3 +411,38 @@ Each connection is also accessible via the ``doctrine.dbal.[name]_connection``
 service where ``[name]`` is the name of the connection.
 
 .. _DBAL documentation: http://docs.doctrine-project.org/projects/doctrine-dbal/en/latest/reference/configuration.html
+
+Shortened Configuration Syntax
+------------------------------
+
+When you are only using one entity manager, all config options available
+can be placed directly under ``doctrine.orm`` config level. 
+
+.. code-block:: yaml
+
+    doctrine:
+        orm:
+            # ...
+            query_cache_driver:
+               # ...
+            metadata_cache_driver:
+                # ...
+            result_cache_driver:
+                # ...
+            connection: ~
+            class_metadata_factory_name:  Doctrine\ORM\Mapping\ClassMetadataFactory
+            default_repository_class:  Doctrine\ORM\EntityRepository
+            auto_mapping: false
+            hydrators:
+                # ...
+            mappings:
+                # ...
+            dql:
+                # ...
+            filters:
+                # ...
+
+This shortened version is commonly used in other documentation sections.
+Keep in mind that you can't use both syntaxes at the same time.
+
+.. _`DQL User Defined Functions`: http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/cookbook/dql-user-defined-functions.html