net/http/fcgi: don't mention threads in docs
net/http/fcgi: don't mention threads in docs

Fixes issue 2942

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5650049
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go