minor #1573 Add a description of global.flashes (chihiro-adachi)

This PR was merged into the 2.2.x-dev branch.

Discussion
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Add a description of global.flashes

global.flashes is available from symfony 3.3, I added it.
https://symfony.com/blog/new-in-symfony-3-3-improved-flash-messages

Commits
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dbae0030 Add a description of global.flashes

doc/providers/twig.rst

@@ -164,6 +164,9 @@ It gives access to the following methods:
     {# The debug flag #}
     {{ global.debug }}
 
+    {# The flash messages (Symfony 3.3 or later) #}
+    {{ global.flashes }}
+
 Rendering a Controller
 ~~~~~~~~~~~~~~~~~~~~~~