USB: small update to Documentation/usb/acm.txt

this docu update mentions that cell phones also use cdc-acm.

Signed-off-by: Oliver Neukum <oliver@neukum.name>

diff --git a/Documentation/usb/acm.txt b/Documentation/usb/acm.txt
index 737d610..17f5c2e 100644
--- a/Documentation/usb/acm.txt
+++ b/Documentation/usb/acm.txt
@@ -46,6 +46,10 @@
 
 	3Com USR ISDN Pro TA
 
+  Some cell phones also connect via USB. I know the following phones work:
+
+	SonyEricsson K800i
+
   Unfortunately many modems and most ISDN TAs use proprietary interfaces and
 thus won't work with this drivers. Check for ACM compliance before buying.