serial: 8250: describe CONFIG_SERIAL_8250_RSA

CONFIG_SERIAL_8250_RSA has waited for a long
time to have meaningful help text so let's
finally describe what this option actually does.

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/Kconfig

@@ -262,7 +262,12 @@ config SERIAL_8250_RSA
 	bool "Support RSA serial ports"
 	depends on SERIAL_8250_EXTENDED
 	help
-	  ::: To be written :::
+	  Say Y here if you have a IODATA RSA-DV II/S ISA card and
+	  would like to use its >115kbps speeds.
+	  You will need to provide module parameter "probe_rsa", or boot-time
+	  parameter 8250.probe_rsa with I/O addresses of this card then.
+
+	  If you don't have such card, or if unsure, say N.
 
 config SERIAL_8250_ACORN
 	tristate "Acorn expansion card serial port support"