docs: fill out rawio description

The original doc entry for rawio didn't mention the values it could
have, the default, or the fact that setting it to "yes" for one disk
effectively set it to "yes" for all disks in the domain.

docs/formatdomain.html.in

@@ -1097,9 +1097,14 @@
         actual raw devices, never for individual partitions or LVM
         partitions (in those cases, the kernel will reject the generic
         SCSI commands, making it identical to device='disk').
-        The optional <code>rawio</code> attribute indicates that the disk
-        is desirous of rawio capability. This attribute is only valid when
-        device is "lun".
+        The optional <code>rawio</code> attribute
+        (<span class="since">since 0.9.10</span>) indicates whether
+        the disk is needs rawio capability; valid settings are "yes"
+        or "no" (default is "no"). If any one disk in a domain has
+        rawio='yes', rawio capability will be enabled for all disks in
+        the domain (because, in the case of QEMU, this capability can
+        only be set on a per-process basis). This attribute is only
+        valid when device is "lun".
         The optional <code>snapshot</code> attribute indicates the default
         behavior of the disk during disk snapshots: "internal"
         requires a file format such as qcow2 that can store both the