docs: domain: document blkiotune {read, write}_{bytes, iops}_sec

Added with commit 3b431929 in v1.2.2 but never documented

https://bugzilla.redhat.com/show_bug.cgi?id=1313613

docs/formatdomain.html.in

@@ -991,6 +991,10 @@
     <device>
       <path>/dev/sdb</path>
       <weight>500</weight>
+      <read_bytes_sec>10000</read_bytes_sec>
+      <write_bytes_sec>10000</write_bytes_sec>
+      <read_iops_sec>20000</read_iops_sec>
+      <write_iops_sec>20000</write_iops_sec>
     </device>
   </blkiotune>
   ...
@@ -1025,8 +1029,22 @@
         absolute path of the device, and <code>weight</code> giving
         the relative weight of that device, in the range [100,
         1000]. After kernel 2.6.39, the value could be in the
-        range [10, 1000].<span class="since">Since 0.9.8</span></dd>
-    </dl>
+        range [10, 1000]. <span class="since">Since 0.9.8</span><br/>
+        Additionally, the following optional sub-elements can be used:
+        <dl>
+          <dt><code>read_bytes_sec</code></dt>
+          <dd>Read throughput limit in bytes per second.
+            <span class="since">Since 1.2.2</span></dd>
+          <dt><code>write_bytes_sec</code></dt>
+          <dd>Write throughput limit in bytes per second.
+            <span class="since">Since 1.2.2</span></dd>
+          <dt><code>read_iops_sec</code></dt>
+          <dd>Read I/O operations per second limit.
+            <span class="since">Since 1.2.2</span></dd>
+          <dt><code>write_iops_sec</code></dt>
+          <dd>Write I/O operations per second limit.
+            <span class="since">Since 1.2.2</span></dd>
+      </dl></dd></dl>
 
 
     <h3><a name="resPartition">Resource partitioning</a></h3>