minor #11371 [Cache] Updated invalid pdo service name (Nyholm)

This PR was merged into the 4.2 branch.

Discussion
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[Cache] Updated invalid pdo service name

This will fix #8406

This is similar to #11370. I made two PRs because PDO was not introduced in 3.4. I think this will be easier to merge.

See https://github.com/symfony/symfony/blob/4.2/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php#L1640

Commits
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fbb3432 [Cache] Updated invalid pdo service name

Author: OskarStark

reference/configuration/framework.rst

@@ -2016,7 +2016,8 @@ default_pdo_provider
 **type**: ``string`` **default**: ``doctrine.dbal.default_connection``
 
 The service id of the database connection, which should be either a PDO or a
-Doctrine DBAL instance.
+Doctrine DBAL instance. The provider is available as the ``cache.default_pdo_provider``
+service.
 
 pools
 .....