minor #15263 [Translation] Update translation.rst (noniagriconomie)

This PR was submitted for the 5.2 branch but it was merged into the 5.3 branch instead.

Discussion
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[Translation] Update translation.rst

Related to https://github.com/symfony/symfony/blob/5.x/src/Symfony/Contracts/Translation/TranslatorInterface.php#L24
I propose to add this here, as not all projects uses ICU translations, so documenting the special `%count%` could be useful

Commits
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b8debc5 Update translation.rst

Author: javiereguiluz

translation.rst

@@ -410,6 +410,15 @@ The ``trans`` filter can be used to translate *variable texts* and complex expre
         {{ message|trans|raw }}
         {{ '<h3>bar</h3>'|trans|raw }}
 
+.. tip::
+
+    When a number is provided as a parameter named "%count%", the message is parsed for plural
+    forms and a translation is chosen according to this number using the following rules:
+
+    .. code-block:: twig
+
+       {{ message|trans({'%name%': 'Fabien', '%count%' => 1}, 'app') }}
+
 .. tip::
 
     You can set the translation domain for an entire Twig template with a single tag: