docs: Add version info for 'backingStoreInput' in domaincaps

Add 'since 5.10' as commonly used in formatdomain to avoid
misunderstandings if element is not present (Is it not supported
because of my version or because of my environment?)
Signed-off-by: NSebastian Mitterle <smitterl@redhat.com>
Reviewed-by: NPeter Krempa <pkrempa@redhat.com>

docs/formatdomaincaps.html.in

@@ -562,6 +562,7 @@
     <p>Reports whether the hypervisor will obey the &lt;backingStore&gt;
     elements configured for a &lt;disk&gt; when booting the guest, hotplugging
     the disk to a running guest, or similar.
+    <span class="since">(Since 5.10)</span>
     </p>
 
     <h4><a id="featureBackup">backup</a></h4>