[FrameworkBundle] Document secret rename logic

Author: noniagriconomie

configuration/secrets.rst

@@ -98,6 +98,10 @@ in ``config/secrets/prod``. You can also set the secret in a few other ways:
     # or let Symfony generate a random value for you
     $ php bin/console secrets:set REMEMBER_ME --random
 
+.. note::
+
+    If you need to rename a secret, you must create the new one and remove the old one.
+
 Referencing Secrets in Configuration Files
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