syscall: fix and document uses of RegEnumKeyEx

RegEnumKeyEx has an undocumented requirement that subsequent calls need
to be made from the same thread. This change documents that requirement
and fixes uses of it in std.

Fixes #49320.

Change-Id: I6d182227e22dd437996a878b3a978943f01b2d61
Reviewed-on: https://go-review.googlesource.com/c/go/+/361154


Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>