Merge branch '6.4' into 7.1

* 6.4:
  Minor tweaks
  [Mercure] Adding hint for multiple topics

Author: javiereguiluz

mercure.rst

@@ -309,6 +309,12 @@ as patterns:
     }
     </script>
 
+However, on the client side (i.e. in JavaScript's ``EventSource``), there is no
+built-in way to know which topic a certain message originates from. If this (or
+any other meta information) is important to you, you need to include it in the
+message's data (e.g. by adding a key to the JSON, or a ``data-*`` attribute to
+the HTML).
+
 .. tip::
 
     Test if a URI Template matches a URL using `the online debugger`_