net/http/cookiejar: mention go.net/publicsuffix.

R=golang-dev, r, gary.burd, dr.volker.dobler
CC=golang-dev
https://codereview.appspot.com/8660046
doc/reference.html: update list of sub-repositories.

R=golang-dev, bradfitz, r
CC=golang-dev
https://codereview.appspot.com/8766043