Deprecate auto picking the first provider

Author: javiereguiluz

security/multiple_user_providers.rst

@@ -82,10 +82,16 @@ a new provider that chains the two together:
             ),
         ));
 
-Now, all firewalls without an explicitly configured user provider will use
-the ``chain_provider`` since it's the first specified. The ``chain_provider``
-will, in turn, try to load the user from both the ``in_memory`` and ``user_db``
-providers.
+Now, all firewalls that explicitly define ``chain_provider`` as their user
+provider will, in turn, try to load the user from both the ``in_memory`` and
+``user_db`` providers.
+
+.. versionadded:: 3.4
+    In previous Symfony versions, firewalls that didn't define their user provider
+    explicitly, used the first existing provider (``chain_provider`` in this
+    example). However, auto-selecting the first user provider has been deprecated
+    in Symfony 3.4 and will throw an exception in 4.0. Always define the provider
+    used by the firewall when there are multiple providers.
 
 You can also configure the firewall or individual authentication mechanisms
 to use a specific provider. Again, unless a provider is specified explicitly,