repmgr: "witness register" - check connection is to primary node

Previously, if the witness server connection details were provided
to "repmgr witness register" rather than those of the primary server,
repmgr a) write the node record to the witness server rather than
the primary, and b) would loop indefinitely trying to copy the
node table to itself.

Addresses GitHub #538.

doc/appendix-release-notes.sgml

@@ -137,6 +137,13 @@ REPMGRD_OPTS="--daemonize=false"</programlisting>
             </para>
           </listitem>
 
+          <listitem>
+            <para>
+              &repmgr;: when executing <command><link linkend="repmgr-witness-register">repmgr witness register</link></command>,
+              chech the node to connected is actually the primary (i.e. not the witness server).  (GitHub #528).
+            </para>
+          </listitem>
+
          <listitem>
             <para>
               &repmgr;: when executing <link linkend="repmgr-standby-switchover"><command>repmgr standby switchover</command></link>,
@@ -315,7 +322,6 @@ REPMGRD_OPTS="--daemonize=false"</programlisting>
             </para>
           </listitem>
 
-
           <listitem>
             <para>
               <application>repmgrd</application>: fix parsing of <option>-d/--daemonize</option> option.

doc/repmgr-witness-register.sgml

@@ -34,6 +34,14 @@
       witness node's <filename>repmgr.conf</filename>, unless these are explicitly
       provided as command line options.
     </para>
+
+    <note>
+      <para>
+        The primary server must be registered with <command><link linkend="repmgr-primary-register">repmgr primary register</link></command> before the witness
+        server can be registered.
+      </para>
+    </note>
+
     <para>
       Execute with the <option>--dry-run</option> option to check what would happen
       without actually registering the witness server.