config: Add detail to auto_join_rooms comment

changelog.d/9291.doc

@@ -0,0 +1 @@
+Add note to `auto_join_rooms` config option explaining existing rooms must be publicly joinable.

docs/sample_config.yaml

@@ -1320,6 +1320,8 @@ account_threepid_delegates:
 # By default, any room aliases included in this list will be created
 # as a publicly joinable room when the first user registers for the
 # homeserver. This behaviour can be customised with the settings below.
+# If the room already exists, make certain it is a publicly joinable
+# room. The join rule of the room must be set to 'public'.
 #
 #auto_join_rooms:
 #  - "#example:example.com"

synapse/config/registration.py

@@ -378,6 +378,8 @@ def generate_config_section(self, generate_secrets=False, **kwargs):
         # By default, any room aliases included in this list will be created
         # as a publicly joinable room when the first user registers for the
         # homeserver. This behaviour can be customised with the settings below.
+        # If the room already exists, make certain it is a publicly joinable
+        # room. The join rule of the room must be set to 'public'.
         #
         #auto_join_rooms:
         #  - "#example:example.com"