DQL custom functions on doctrine reference page

Custom DQL functions can be configured in two ways, according to what I was told [ here ] (https://github.com/symfony/symfony-docs/pull/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)
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