serial: document start_rx member at struct uart_ops

Fix this doc build warning:
	./include/linux/serial_core.h:397: warning: Function parameter or member 'start_rx' not described in 'uart_ops'
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/5d07ae2eec8fbad87e623160f9926b178bef2744.1660829433.git.mchehab@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/serial_core.h

@@ -141,6 +141,14 @@ struct gpio_desc;
  *	Locking: none.
  *	Interrupts: caller dependent.
  *
+ * @start_rx: ``void ()(struct uart_port *port)``
+ *
+ *	Start receiving characters.
+ *
+ *	Locking: @port->lock taken.
+ *	Interrupts: locally disabled.
+ *	This call must not sleep
+ *
  * @stop_rx: ``void ()(struct uart_port *port)``
  *
  *	Stop receiving characters; the @port is in the process of being closed.