net/http/cookiejar: document format of domain in PublicSuffix
net/http/cookiejar: document format of domain in PublicSuffix

Document what values a PublicSuffixList must accept as
a domain in a call to PublicSuffix.

R=bradfitz, nigeltao
CC=golang-codereviews
https://codereview.appspot.com/47560044

Committer: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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