[Mercure] Adding hint for multiple topics

Page: https://symfony.com/doc/6.4/mercure.html#subscribing

Closes #20574
Closes symfony/mercure-bundle#71
Closes symfony/mercure-bundle#82

Info is taken from #20574 (comment)

mercure.rst

@@ -309,6 +309,11 @@ as patterns:
     }
     </script>
 
+However, on the client side (i.e. in JavaScript's ``EventSource``), there is no built-in way
+to see which topic a certain message is coming from. So 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`_