windows: document safe usage of TrusteeValue

Failure to pin the memory pointed to by a TrusteeValue (a uintptr)
can lead to the GC collecting the memory while it is still being
referenced.

Fixes #79923

Change-Id: Ie28884856298cd60693c903843be6aa06a6a6964
Reviewed-on: https://go-review.googlesource.com/c/sys/+/789000
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>