runtime: update go:notinheap documentation
runtime: update go:notinheap documentation

The rules for go:notinheap were recently tweaked to disallow stack
allocation (CL 249917). This CL updates the documentation about
go:notinheap in runtime/HACKING.md.

Change-Id: Ibca5d9b9d02e1c22c6af1d303aa84c6303a86d92
Reviewed-on: https://go-review.googlesource.com/c/go/+/257357
Trust: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go,cloudflare/go,CAFxX/go,cloudflare/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,golang/go,golang/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,golang/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go,nawawi/go,golang/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go