e1000e Kconfig: remove ref to nonexistant docs

There is no Documentation/networking/e1000e.txt.

Signed-off-by: Jason Uhlenkott <jasonuhl@jasonuhl.org>
Cc: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>