doc: update repmgrd log rotation configuration

In the sample logrotate configuration file, use "copytruncate" rather than "create",
as repmgrd currently doesn't reopen the log file (unless the configuration changes).

Per suggestion in GitHub #465.

doc/repmgrd-configuration.sgml

@@ -334,14 +334,39 @@ REPMGRD_ENABLED=no
    Sample configuration to rotate logfiles weekly with retention for
    up to 52 weeks and rotation forced if a file grows beyond 100Mb:
    <programlisting>
-    /var/log/postgresql/repmgr-9.6.log {
+    /var/log/repmgr/repmgrd.log {
         missingok
         compress
         rotate 52
         maxsize 100M
         weekly
-        create 0600 postgres postgres
+        copytruncate
     }</programlisting>
   </para>
+
+  <note>
+    <para>
+      Currently <application>repmgrd</application> will not reopen the log
+      file unless the configuration is reloaded via <literal>SIGHUP</literal>
+      <emphasis>and</emphasis> the configuration changes.
+    </para>
+    <para>
+      Therefore it's recommended to use <literal>copytruncate</literal> when
+      rotating logs, so that the contents of the existing file are rotated out
+      by creating a copy, then truncating the original file, which
+      <application>repmgrd</application> still holds open.
+    </para>
+    <para>
+      Note that any logging data written in the (short) timespan after the existing file
+      is copied, but before it is truncated, will be lost; see the
+      <ulink url="https://linux.die.net/man/8/logrotate">logrotate documentation</ulink>.
+    </para>
+    <para>
+      This restriction may be lifted in a future &repmgr; version by providing a
+      signal which causes <application>repmgrd</application> to reopen the log file,
+      and which can be send from <command>logrotate</command> via a <command>postrotate</command>
+      script.
+    </para>
+  </note>
  </sect1>
 </chapter>