Merge branch '3.4' into 4.4

* 3.4:
  [Yaml] fix dumping strings containing CRs
  [DI] Fix XmlFileLoader bad error message
  Tweak message
  improve PlaintextPasswordEncoder docBlock summary
  [Validator] Add two missing translations for the Arabic (ar) locale
  Use some PHP 5.4 constants unconditionally
  Revert "bug symfony#28179 [DomCrawler] Skip disabled fields processing in Form"
  Add Spanish translation
  Fix typo
  [Validator] add Japanese translation
  Fix typo
  Add Polish translation
  [SecurityBundle] Minor fixes in configuration tree builder
  bumped Symfony version to 3.4.39
  updated VERSION for 3.4.38
  update CONTRIBUTORS for 3.4.38
  updated CHANGELOG for 3.4.38

Author: nicolas-grekas

Encoder/PlaintextPasswordEncoder.php

@@ -14,7 +14,9 @@
 use Symfony\Component\Security\Core\Exception\BadCredentialsException;
 
 /**
- * PlaintextPasswordEncoder does not do any encoding.
+ * PlaintextPasswordEncoder does not do any encoding but is useful in testing environments.
+ *
+ * As this encoder is not cryptographically secure, usage of it in production environments is discouraged.
  *
  * @author Fabien Potencier <fabien@symfony.com>
  */