Update mercure.rst regarding JWT token secret

👋 

The documentation for the configuration of mercure seems quite wrong: it has a big "caution" block stating that the `MERCURE_JWT_SECRET` should contain an _actual JWT_ ... instead of a secret, which is weird (and definitely not how it works).

I propose to remove these (maybe out of date?) parts to prevent further confusion (_I spent a full day examining the actual source to understand what was really needed in the env var_).

Also, added a tip on how setting the cookies twice would not work.

PS: I created this PR against the 5.3 (current) branch since the 4.4 branch does not have the same paragraphs. Hope it's good.

Author: tchapi

mercure.rst

@@ -111,38 +111,7 @@ the publicly available URL (e.g. ``https://example.com/.well-known/mercure``).
 The clients must also bear a `JSON Web Token`_ (JWT)
 to the Mercure Hub to be authorized to publish updates and, sometimes, to subscribe.
 
-This JWT should be stored in the ``MERCURE_JWT_SECRET`` environment variable.
-
-The JWT must be signed with the same secret key as the one used by
-the Hub to verify the JWT (``!ChangeMe!`` in you use the Local Web Server or
-Symfony Docker).
-Its payload must contain at least the following structure to be allowed to
-publish:
-
-.. code-block:: json
-
-    {
-        "mercure": {
-            "publish": []
-        }
-    }
-
-Because the array is empty, the Symfony app will only be authorized to publish
-public updates (see the authorization_ section for further information).
-
-.. tip::
-
-    The jwt.io website is a convenient way to create and sign JWTs.
-    Checkout this `example JWT`_, that grants publishing rights for all *topics*
-    (notice the star in the array).
-    Don't forget to set your secret key properly in the bottom of the right panel of the form!
-
-.. caution::
-
-    Don't put the secret key in ``MERCURE_JWT_SECRET``, it will not work!
-    This environment variable must contain a JWT, signed with the secret key.
-
-    Also, be sure to keep both the secret key and the JWTs... secrets!
+This token must be signed with the same secret key as the one used by the Hub to verify the JWT (``!ChangeMe!`` in you use the Local Web Server or Symfony Docker), which should be stored in the ``MERCURE_JWT_SECRET`` environment variable.
 
 If you don't want to use the provided environment variables,
 use the following configuration:
@@ -482,6 +451,14 @@ And here is the controller::
         }
     }
 
+
+.. tip::
+
+    You cannot use the ``mercure()`` helper and the ``setCookie()``
+    method at the same time (it would set the cookie twice on a single request). Choose
+    either one method or the other.
+
+
 Programmatically Generating The JWT Used to Publish
 ---------------------------------------------------