docs/memory: Explictly state that MemoryRegion priority is signed

When memory regions overlap, priority can be used to specify
which of them takes priority. By making the priority values signed
rather than unsigned, we make it more convenient to implement
a situation where one "background" region should appear only
where no other region exists: rather than having to explicitly
specify a high priority for all the other regions, we can let them take
the default (zero) priority and specify a negative priority for the
background region.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Acked-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMarcel Apfelbaum <marcel.a@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

docs/memory.txt

@@ -80,6 +80,10 @@ guest.  This is done with memory_region_add_subregion_overlap(), which
 allows the region to overlap any other region in the same container, and
 specifies a priority that allows the core to decide which of two regions at
 the same address are visible (highest wins).
+Priority values are signed, and the default value is zero. This means that
+you can use memory_region_add_subregion_overlap() both to specify a region
+that must sit 'above' any others (with a positive priority) and also a
+background region that sits 'below' others (with a negative priority).
 
 Visibility
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