merged branch gbirke/form_error_docs (PR #1599)

Commits
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22a49f1 Better docstring for FormError constructor

Discussion
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Better docstring for FormError constructor

Better docs for placeholder format of FormError.

src/Symfony/Component/Form/FormError.php

@@ -33,6 +33,10 @@ class FormError
     /**
      * Constructor
      *
+     * Any array key in $messageParameters will be used as a placeholder in
+     * $messageTemplate.
+     * @see Symfony\Component\Translation\Translator
+     *
      * @param string $messageTemplate      The template for the error message
      * @param array $messageParameters     The parameters that should be
      *                                     substituted in the message template.