unsafe: document that unsafe programs are not protected

unsafe: document that unsafe programs are not protected
The compatibility guideline needs to be clear about this even
though it means adding a clause that was not there from the
beginning. It has always been understood, so this isn't really
a change in policy, just in its expression.

LGTM=bradfitz, gri, rsc
R=golang-codereviews, bradfitz, gri, rsc
CC=golang-codereviews
https://golang.org/cl/162060043