Documentation: FULL_DUPLEX_MAGIC isn't a magic number

It's used to control a mysterious register on the DEC DE21040,
see comment in drivers/net/ethernet/dec/tulip/de2104x.c

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Link: https://lore.kernel.org/r/ff28a15f9154589788277807523aa71c45c24d28.1663280877.git.nabijaczleweli@nabijaczleweli.xyz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>