docs: Clarify semantics of model/@usable attribute in dom caps
docs: Clarify semantics of model/@usable attribute in dom caps

The documentation could confuse people to expect that CPU models with
usable='no' attribute are not usable at all on the current host. But
they cannot be only used without explicitly disabling some features.

Signed-off-by: Jiri Denemark <62bdf77dc47919a4d59a91822129d14633cfca81@redhat.com>
Reviewed-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>

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