cmd: improve description of the cdp command

Users might not know what 'CDP' refers to. Provide basic information.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index b2d0348..1d7ddb4 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -2016,7 +2016,9 @@
 config CMD_CDP
 	bool "cdp"
 	help
-	  Perform CDP network configuration
+	  The cdp command is used to announce the U-Boot device in the network
+	  and to retrieve configuration data including the VLAN id using the
+	  proprietary Cisco Discovery Protocol (CDP).
 
 config CMD_SNTP
 	bool "sntp"