Added a note about using slashes and the special _format param

Author: javiereguiluz

routing/slash_in_parameter.rst

@@ -77,6 +77,15 @@ a more permissive regular expression for it:
 
 That's it! Now, the ``{token}`` parameter can contain the ``/`` character.
 
+.. note::
+
+    If the route includes the special ``{_format}`` placeholder, you shouldn't
+    use the ``.+`` requirement for the parameters that allow slashes. For example,
+    if the pattern is ``/share/{token}.{_format}`` and ``{token}`` allows any
+    character, the ``/share/foo/bar.json`` URL will consider ``foo/bar.json``
+    as the token and the format will be empty. This can be solved replacing the
+    ``.+`` requirement by ``[^.]+`` to allow any character except dots.
+
 .. note::
 
     If the route defines several placeholders and you apply this permissive