minor #10281 Update BCryptPasswordEncoder.php (Szymciosek)

This PR was submitted for the 2.3-dev branch but it was merged into the 2.3 branch instead (closes #10281).

Discussion
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Update BCryptPasswordEncoder.php

Commits
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4c81fb3 Update BCryptPasswordEncoder.php

src/Symfony/Component/Security/Core/Encoder/BCryptPasswordEncoder.php

@@ -29,6 +29,7 @@ class BCryptPasswordEncoder extends BasePasswordEncoder
      *
      * @param integer $cost The algorithmic cost that should be used
      *
+     * @throws \RuntimeException When no BCrypt encoder is available
      * @throws \InvalidArgumentException if cost is out of range
      */
     public function __construct($cost)