sync/atomic: document that users must deal with 64-bit alignment
sync/atomic: document that users must deal with 64-bit alignment

Update issue 599.

R=dvyukov, iant, minux.ma
CC=golang-dev
https://codereview.appspot.com/7001056

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