docs: Document support for obeying <backingStore> of <disk> on input

docs: Document support for obeying <backingStore> of <disk> on input

Until now we've only supported <backingStore> in an output mode. The
documentation for the element states that hypervisor drivers may start
to obey it in the future.

Update the documentation so that it mentions the recently added
'backingStoreInput' domain capability and explain what happens if it is
supported and <backingStore> is present on input.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>