Make it clearer that a witness node counts as a "sibling node"

It's not attached to the primary per-se, but needs to know what
the current primary is in order to correctly synchronise its
copy of the metadata.

Per GitHub #560.

doc/repmgr-standby-promote.sgml

@@ -37,7 +37,8 @@
     <para>
       Beginning with <link linkend="release-4.4">repmgr 4.4</link>,
       the option <option>--siblings-follow</option> can be used to have
-      all other standbys follow the new primary.
+      all other standbys (and a witness server, if in use)
+	  follow the new primary.
     </para>
 
     <note>
@@ -119,6 +120,11 @@
             Have all sibling nodes (nodes formerly attached to the same upstream
             node as the promotion candidate) follow this node after it has been promoted.
           </para>
+		  <para>
+			Note that a witness server, if in use, is also
+			counted as a &quot;sibling node&quot; as it needs to be instructed to
+			synchronise its metadata with the new primary.
+		  </para>
           <important>
             <para>
               Do <emphasis>not</emphasis> provide this option when configuring

doc/repmgr-standby-switchover.sgml

@@ -26,7 +26,9 @@
       these to follow the new primary if the option <literal>--siblings-follow</literal>
       is specified. This requires a passwordless SSH connection between the promotion
       candidate (new primary) and the nodes attached to the demotion candidate
-      (existing primary).
+      (existing primary). Note that a witness server, if in use, is also
+	  counted as a &quot;sibling node&quot; as it needs to be instructed to
+	  synchronise its metadata with the new primary.
     </para>
     <note>
       <para>