Fixed the explanation of the utf8 router option

Author: javiereguiluz

reference/configuration/framework.rst

@@ -1152,7 +1152,10 @@ utf8
 
 **type**: ``boolean`` **default**: ``false``
 
-When this option is set to ``true``, route patterns can include UTF-8 characters.
+When this option is set to ``true``, the regular expressions used in the
+:ref:`requirements of route parameters <routing-requirements>` will match any
+UTF-8 character when using ``.``, instead of matching only a single byte.
+
 If the charset of your application is UTF-8 (as defined in the
 :ref:`getCharset() method <configuration-kernel-charset>` of your kernel) it's
 recommended to set it to ``true``. This will make non-UTF8 URLs to generate 404