add a comment about alias interfaces and primary interface of scopes

(intended to fix Debian wishlist bug #179780 and normal bug #64458)

man/en_US/ifconfig.8

-.TH IFCONFIG 8 "2002-07-04" "net-tools" "Linux Programmer's Manual"
+.TH IFCONFIG 8 "2003-02-08" "net-tools" "Linux Programmer's Manual"
 .SH NAME
 ifconfig \- configure a network interface
 .SH SYNOPSIS
@@ -54,7 +54,14 @@ be more verbose for some error conditions
 The name of the interface.  This is usually a driver name followed by
 a unit number, for example
 .B eth0
-for the first Ethernet interface.
+for the first Ethernet interface. If your kernel supports alias interfaces,
+you can specify them with 
+.B eth0:0
+for the first alias of eth0. You can use them to assign a second address. To
+delete an alias interface use
+.B ifconfig eth0:0 down
+. Note: for every scope (i.e. same net with address/netmask combination) all
+aliases are deleted, if you delete the first (primary).
 .TP
 .B up
 This flag causes the interface to be activated.  It is implicitly