ALSA: Add description of new fm_port option for snd-es1688 driver

Signed-off-by: NTakashi Iwai <tiwai@suse.de>

Documentation/sound/alsa/ALSA-Configuration.txt

@@ -609,6 +609,7 @@ Prior to version 0.9.0rc4 options had a 'snd_' prefix. This was removed.
     Module for ESS AudioDrive ES-1688 and ES-688 sound cards.
 
     port	- port # for ES-1688 chip (0x220,0x240,0x260)
+    fm_port	- port # for OPL3 (option; share the same port as default)
     mpu_port	- port # for MPU-401 port (0x300,0x310,0x320,0x330), -1 = disable (default)
     irq		- IRQ # for ES-1688 chip (5,7,9,10)
     mpu_irq	- IRQ # for MPU-401 port (5,7,9,10)