feature #6433 Documented the detectCorrupted and corruptedMessage options (javiereguiluz)

This PR was squashed before being merged into the master branch (closes #6433).

Discussion
----------

Documented the detectCorrupted and corruptedMessage options

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | master (3.1+)
| Fixed tickets | symfony/symfony#17458

Commits
-------

2140822 Documented the detectCorrupted and corruptedMessage options

Author: wouterj

reference/constraints/Image.rst

@@ -24,6 +24,7 @@ of the documentation on this constraint.
 |                | - `allowSquare`_                                                      |
 |                | - `allowLandscape`_                                                   |
 |                | - `allowPortrait`_                                                    |
+|                | - `detectCorrupted`_                                                  |
 |                | - `mimeTypesMessage`_                                                 |
 |                | - `sizeNotDetectedMessage`_                                           |
 |                | - `maxWidthMessage`_                                                  |
@@ -35,6 +36,7 @@ of the documentation on this constraint.
 |                | - `allowSquareMessage`_                                               |
 |                | - `allowLandscapeMessage`_                                            |
 |                | - `allowPortraitMessage`_                                             |
+|                | - `corruptedMessage`_                                                 |
 |                | - See :doc:`File </reference/constraints/File>` for inherited options |
 +----------------+-----------------------------------------------------------------------+
 | Class          | :class:`Symfony\\Component\\Validator\\Constraints\\Image`            |
@@ -311,6 +313,18 @@ allowPortrait
 
 If this option is false, the image cannot be portrait oriented.
 
+detectCorrupted
+~~~~~~~~~~~~~~~
+
+.. versionadded:: 3.1
+     The ``detectCorrupted`` option was introduced in Symfony 3.1.
+
+**type**: ``boolean`` **default**: ``false``
+
+If this option is true, the image contents are validated to ensure that the
+image is not corrupted. This validation is done with PHP's :phpfunction:`imagecreatefromstring`
+function, which requires the `PHP GD extension`_ to be enabled.
+
 sizeNotDetectedMessage
 ~~~~~~~~~~~~~~~~~~~~~~
 
@@ -392,4 +406,16 @@ Portrait oriented images are not allowed``
 
 The error message if the image is portrait oriented and you set `allowPortrait`_ to ``false``.
 
+corruptedMessage
+~~~~~~~~~~~~~~~~
+
+.. versionadded:: 3.1
+     The ``corruptedMessage`` option was introduced in Symfony 3.1.
+
+**type**: ``string`` **default**: ``The image file is corrupted.``
+
+The error message when the `detectCorrupted`_ option is enabled and the image
+is corrupted.
+
 .. _`IANA website`: http://www.iana.org/assignments/media-types/image/index.html
+.. _`PHP GD extension`: http://php.net/manual/en/book.image.php