Update mercure.rst

Co-authored-by: Saif Eddin Gmati <29315886+azjezz@users.noreply.github.com>

Author: dunglas

mercure.rst

@@ -116,7 +116,7 @@ MercureBundle provides a more advanced configuration configuration:
 
 * ``secret``: the key to use to sign the JWT
 * ``publish``: a list of topics to allow publishing to when generating the JWT
-* ``subscribe``: a list of topics to allow subscribing to when generating the JWT
+* ``subscribe``: a list of topics to allow subscribing to when generating the JWT (only usable when `secret`, or `factory` are provided )
 * ``algorithm``: The algorithm to use to sign the JWT (only usable when `secret` is provided )
 * ``provider``: The ID of a service to call to provide the JWT
 * ``factory``: The ID of a service to call to create the JWT (all other options, beside `subscribe`, and `publish` will be ignored)