A pointer type of underlying type unsafe.Pointer can be used in
unsafe conversions. Document unfortunate status quo.

Fixes #19306.

Change-Id: I28172508a200561f8df366bbf2c2807ef3b48c97
Reviewed-on: https://go-review.googlesource.com/42132Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>