runtime: warn that KeepAlive is not an unsafe.Pointer workaround

Even experienced users occasionally mistake that runtime.KeepAlive can
be used as a workaround for following the unsafe.Pointer safety rules,
but it cannot. Add an explicit warning to this effect to dissuade
users from trying to use it as such.

Fixes #47562.

Change-Id: I842e33a3e1c080933c6b1bd1b6318448adbf495c
Reviewed-on: https://go-review.googlesource.com/c/go/+/340269


Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>