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

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang