DQL custom functions on doctrine reference page

Seems that there is two possibilities to configure custom DQL functions

It should be clearified in doctrine reference page because It may confuse some people if they just read the cookbook: http://symfony.com/doc/current/cookbook/doctrine/custom_dql_functions.html
Update doctrine.rst
Update doctrine.rst
Fixed broked link
Shorten syntax mention on doctrine reference

Changed documentation to explain shorten syntax in general, not just DQL functions part
Update doctrine.rst

Made more clear the shorten doctrine.yml syntax, leaving  only the first level of configuration.
Update doctrine.rst

Proposed changes to format
Update doctrine.rst

Minor typos changes
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More typos oops :P
Update doctrine.rst

Fix for "Title underline too short"
Update doctrine.rst

Included more suggestions
feature #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 #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 ] (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)

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