KVM: arm64: Document default vPMU behavior on heterogeneous systems

KVM maintains a mask of supported CPUs when a vPMU type is explicitly
selected by userspace and is used to reject any attempt to run the vCPU
on an unsupported CPU. This is great, but we're still beholden to the
default behavior where vCPUs can be scheduled anywhere and guest
counters may silently stop working.

Avoid confusing the next poor sod to look at this code and document the
intended behavior.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230525212723.3361524-3-oliver.upton@linux.dev