"standby register/follow": provide primary node details for event notifications

For events generated by these commands, it may be useful to know details
of the primary node. This makes following additional parameters available
to event notification scripts:

- %p: node ID of the primary
- %a: node name of the primary
- %c: conninfo string for the primary

Implements GitHub #375

doc/repmgr-standby-follow.sgml

@@ -87,9 +87,14 @@
     <para>
       A <literal>standby_follow</literal> <link linkend="event-notifications">event notification</link> will be generated.
     </para>
+    <para>
+      If provided, &repmgr; will subsitute the placeholders <literal>%p</literal> with the node ID of the primary
+      being followed, <literal>%c</literal> with its <literal>conninfo</literal> string, and
+      <literal>%a</literal> with its node name.
+    </para>
   </refsect1>
 
- <refsect1>
+  <refsect1>
     <title>See also</title>
     <para>
      <xref linkend="repmgr-node-rejoin">