Documentation/acpi: add Windows-specific documentation

When using the Windows fast startup mechanism which is enabled by
default, Windows will use a cached version of the ACPI tables during
normal boots after a clean shutdown. Since I've run into this issue and
spent quite a bit of time debugging the wrong issue due to this, better
document this possibly unexpected behavior.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: Ia9e65f6a3aff13fa54abe68c8f5fcbf9bc6efc1a
Reviewed-on: https://review.coreboot.org/c/coreboot/+/77354
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>

Documentation/acpi/index.md

@@ -11,6 +11,9 @@ upwards.
 
 - [GPIO toggling in ACPI AML](gpio.md)
 
+## Windows-specific ACPI documentation
+
+- [Windows-specific documentation](windows.md)
 
 ##  ACPI specification - Useful links
 

Documentation/acpi/windows.md

@@ -0,0 +1,9 @@
+# Testing ACPI changes under Windows
+
+When testing ACPI changes in coreboot against Windows 8 or newer, beware that
+during a normal boot after a clean shutdown, Windows will use the fast startup
+mechanism which results in it not evaluating the changed ACPI code but instead
+using some cached version which won't include the changes that were supposed to
+be tested. In order for Windows to actually use the new ACPI tables, either
+disable the fast startup or just tell Windows to do a reboot which will make it
+read and use the ACPI tables in memory instead of an outdated cached version.