uio: document uio_hv_generic regions

Describe the regions present with uio_hv_generic in documentation for
driver API.
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/driver-api/uio-howto.rst

@@ -698,6 +698,11 @@ prevents the device from generating further interrupts until the bit is
 cleared. The userspace driver should clear this bit before blocking and
 waiting for more interrupts.
 
+The vmbus device regions are mapped into uio device resources:
+    0) Channel ring buffers: guest to host and host to guest
+    1) Guest to host interrupt signalling pages
+    2) Guest to host monitor page
+
 Further information
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