minor #3608 [Reference][Forms] add versionadded directive for multiple option of file type (xabbuh)

This PR was merged into the master branch.

Discussion
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[Reference][Forms] add versionadded directive for multiple option of file type

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.5+
| Fixed tickets | #3607

Commits
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eefd0ab add versionadded directive for multiple option

Author: weaverryan

reference/forms/types/file.rst

@@ -86,6 +86,9 @@ Field Options
 multiple
 ~~~~~~~~
 
+.. versionadded:: 2.5
+    The ``multiple`` option was introduced in Symfony 2.5.
+
 **type**: ``Boolean`` **default**: ``false``
 
 When set to true, the user will be able to upload multiple files at the same time.