net/smtp: clarify that SendMail's auth param is optional

It wasn't obvious that the Auth could be nil.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/13060048
net/http: document Request.Body more

Fixes issue 6221

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/13289043