minor #35958 improve PlaintextPasswordEncoder docBlock summary (jrush…

…low)

This PR was squashed before being merged into the 3.4 branch.

Discussion
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improve PlaintextPasswordEncoder docBlock summary

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       |
| License       | MIT
| Doc PR        |

Updates class summary as suggested in tkt #35927 & pr #35929 to suggest the encoder is for test usage.

Commits
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622facf Tweak message
a56d262 improve PlaintextPasswordEncoder docBlock summary

src/Symfony/Component/Security/Core/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>
  */