Add doc about unicode requirements

Author: nicolas-grekas

routing/requirements.rst

@@ -159,6 +159,14 @@ Path     Parameters
 ``/es``  *won't match this route*
 =======  ========================
 
+.. note::
+
+    Since Symfony 3.2, requirements can be prefixed by the ``*`` character to
+    enable Unicode matching, thus making ``.`` match any UTF-8 character instead
+    of just a single byte. Unicode matching is also automatically enabled
+    whenever the route or a requirement contains a non-ASCII UTF-8 character or
+    a `PCRE Unicode property`_ (``\p{xx}``, ``\P{xx}`` or ``\X``).
+
 .. tip::
 
     The route requirements can also include container parameters, as explained
@@ -287,3 +295,5 @@ Adding Dynamic Requirements with Expressions
 
 For really complex requirements, you can use dynamic expressions to match *any*
 information on the request. See :doc:`/routing/conditions`.
+
+.. _`PCRE Unicode property`: http://php.net/manual/en/regexp.reference.unicode.php