udhcpc man page should reference /etc/network/interfaces

While using udhcpc as a replacement for dhcp3-client to work around a bug (Bug #351378), I was heading down the path of creating a custom "up" script to invoke udhcpc directly when I happened to notice in the DHCP section of the interfaces man page that it supported udhcpc. The connection between udhcpc and /etc/network/interfaces should be made more explicit by adding a reference to it in the udhcpc man page. (And this should be pushed upstream to Debian.)