minor #20580 [Mercure] Adding hint for multiple topics (ThomasLandauer)

This PR was merged into the 6.4 branch.

Discussion
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[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)

Commits
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3dca32e [Mercure] Adding hint for multiple topics

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`_