Correct comment in NativeSessionStorage regarding session.save_handler

It's save_handler, not save_handlers.
Merge branch '2.1' into 2.2

* 2.1:
  fixed a circular call (closes #6864)
  Correct comment in NativeSessionStorage regarding session.save_handler
  [Security] Add PHPDoc to AuthenticationEvents
Merge branch '2.2'

* 2.2:
  fixed regression in the Finder component (it was possible to use it without using exec before, closes #6357)
  fixed a circular call (closes #6864)
  typo
  [Security] [Tests] added unit tests for the UserPasswordValidator class and made the validator service for the UserPassword constraint configurable.
  fixed wrong indentation
  tweaked previous commit
  [HttpKernel] Fix the URI signer (closes #6801)
  Add Arabic translations.
  [HttpKernel] fixed regression when rendering an inline controller and passing some objects  (closes #6822)
  [FrameworkBundle] fixed typo
  renamed some classes and Twig functions to more descriptive names (refs #6871)
  Classcollectionloader: fix traits + enhancements
  Fix a deprecated method call in the tests
  Update `composer.json` files: - to allow versions ~2.2 (>=2.2,<3.0) of Doctrine DBAL, ORM & Common - fixed Propel1 versions difference between main and bridge files - fixed Twig versions difference between main and bridge files - to allow versions ~1.11 (>=1.11,<2.0) of Twig - fixed Locale ext-intl version to accept all, not non-existing version
  Correct comment in NativeSessionStorage regarding session.save_handler
  [Security] Add PHPDoc to AuthenticationEvents