Enable pg_rewind to be used with PostgreSQL 9.3/9.4

pg_rewind is not part of the core distribution for those, but we
provided support in repmgr 3.3 so should extend it to repmgr 4.

Note that there is no check in place whether the pg_rewind binary
exists, so it's up to the user to ensure it's present.

Addresses GitHub #413.

doc/repmgr-standby-switchover.sgml

@@ -84,11 +84,14 @@
       </varlistentry>
 
       <varlistentry>
-        <term><option>--force-rewind</option></term>
+        <term><option>--force-rewind[=/path/to/pg_rewind]</option></term>
         <listitem>
           <para>
             Use <application>pg_rewind</application> to reintegrate the old primary if necessary
-            (PostgreSQL 9.5 and later).
+            (and the prerequisites for using <application>pg_rewind</application> are met).
+            If using PostgreSQL 9.3 or 9.4, and the <application>pg_rewind</application>
+            binary is not installed in the PostgreSQL <filename>bin</filename> directory,
+            provide its full path. For more details see also <xref linkend="switchover-pg-rewind">.
           </para>
         </listitem>
       </varlistentry>