faq: add entry about nil interfaces vs. nil pointers
faq: add entry about nil interfaces vs. nil pointers

Fixes issue 2778.

R=bradfitz, rsc, iant, adg
CC=golang-dev
http://codereview.appspot.com/5672078

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go