Add note on system IDs to docs

Author: raphaelm

docs/tans.rst

@@ -154,6 +154,13 @@ You SHOULD use this facility together with the client and dialog state restorati
     print(response.status)
     print(response.responses)
 
+System IDs
+----------
+
+During your first connection, the bank will assign your client application a "system ID" accessible
+as ``client.system_id``. Some banks require you to pass the same system ID with your next connection
+with ``FinTS3PinTanClient(..., system_id=stored_system_id)`` to avoid a TAN requirement on every login.
+(This happens automatically when using ``from_data``).
 
 Reference
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