Documentation: KVM: Call out that KVM strictly follows the 8254 PIT spec

Explicitly document that the behavior of KVM_SET_PIT2 strictly conforms
to the Intel 8254 PIT hardware specification, specifically that a write of
'0' adheres to the spec's definition that a programmed count of '0' is
converted to the maximum possible value (2^16).  E.g. an unaware userspace
might attempt to validate that KVM_GET_PIT2 returns the exact state set
via KVM_SET_PIT2, and be surprised when the returned count is 65536, not 0.

Add a references to the Intel 8254 PIT datasheet that will hopefully stay
fresh for some time (the internet isn't exactly brimming with copies of
the 8254 datasheet).

Link: https://lore.kernel.org/all/CANypQFbEySjKOFLqtFFf2vrEe=NBr7XJfbkjQhqXuZGg7Rpoxw@mail.gmail.com
Signed-off-by: Jiaming Zhang <r772577952@gmail.com>
Link: https://lore.kernel.org/r/20250905174736.260694-1-r772577952@gmail.com
[sean: add context Link, drop local APIC change, massage changelog accordingly]
Signed-off-by: Sean Christopherson <seanjc@google.com>

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