doc/go1.3.html: add note about unsafe.Pointer strictness
doc/go1.3.html: add note about unsafe.Pointer strictness

The vet check is in CL 10470044.

LGTM=bradfitz, r
R=r, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/91480044

wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old