Documentation: rs485: Mention uart_get_rs485_mode()

Add to rs485 documentation that serial core prepares the struct
serial_rs485 when uart_get_rs485_mode() is called. Remove the wrong
claim that the driver must fill it by itself.
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20221019093343.9546-4-ilpo.jarvinen@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/driver-api/serial/serial-rs485.rst

@@ -34,8 +34,9 @@ RS485 Serial Communications
    parameters in the platform data and in ioctls.
 
    The device tree can also provide RS485 boot time parameters
-   [#DT-bindings]_. The driver is in charge of filling this data structure
-   from the values given by the device tree.
+   [#DT-bindings]_. The serial core fills the struct serial_rs485 from the
+   values given by the device tree when the driver calls
+   uart_get_rs485_mode().
 
    Any driver for devices capable of working both as RS232 and RS485 should
    implement the rs485_config callback and provide rs485_supported in the
@@ -48,7 +49,7 @@ RS485 Serial Communications
    serial_rs485 structure matching to the current configuration.
 
 .. kernel-doc:: include/uapi/linux/serial.h
-   :identifiers: serial_rs485
+   :identifiers: serial_rs485 uart_get_rs485_mode
 
 4. Usage from user-level
 ========================