doc/effective_go.html: minor updates, part 1

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/7454044
net/http: document net/http/cookiejar on CookieJar

R=golang-dev, nigeltao, adg
CC=golang-dev
https://codereview.appspot.com/7444048
net/http: don't special-case multipart/byteranges responses

Fixes issue 4767

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/7435046