minor #7427 Add apcu cache driver to doctrine config reference (Epskampie)

This PR was submitted for the 3.0 branch but it was merged into the 2.7 branch instead (closes #7427).

Discussion
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Add apcu cache driver to doctrine config reference

apcu cache driver was added in doctrine/cache 1.6:
https://github.com/doctrine/cache/releases/tag/v1.6.0

doctrine/cache version 1.6 was first used in symfony 3.0

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d99e1a5 Add apcu cache driver to doctrine config reference

Author: xabbuh

reference/configuration/doctrine.rst

@@ -297,7 +297,7 @@ certain classes, but those are for very advanced use-cases only.
 Caching Drivers
 ~~~~~~~~~~~~~~~
 
-For the caching drivers you can specify the values ``array``, ``apc``, ``memcache``,
+For the caching drivers you can specify the values ``array``, ``apc``, ``apcu``, ``memcache``,
 ``memcached``, ``redis``, ``wincache``, ``zenddata``, ``xcache`` or ``service``.
 
 The following example shows an overview of the caching configurations: