spec: clarify that any unsafe.Pointer type is okay in conversion
spec: clarify that any unsafe.Pointer type is okay in conversion

The spec is not clear about whether this is allowed or not,
but both compilers allow it, because the reflect implementation
takes advantage of it. Document current behavior.

Fixes issue 4679.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7303064
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go