docs/misc: document PV control/feature keys for laptop/slate mode

docs/misc: document PV control/feature keys for laptop/slate mode

Commit 4c8153d9 "add ACPI device for Windows laptop/slate mode switch"
added code to provide an 'laptop/slate mode' ACPI device to guests.

When present this device is used by Microsoft Windows to bind a HID
driver which controls whether the Windows desktop appearance is optimized
for laptop/desktop or slate/tablet PCs. The mechanism for switching
between modes is to open a handle to this driver and write a byte of
arbitrary data.

This patch documents xenstore keys such that a PV agent running in a
Windows guest can advertise the capability to, and receive instruction
from, a toolstack to cause such a mode switch.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>