standby promote: fall back to "pg_ctl promote" if necessary

From PostgreSQL 12, the SQL-level function "pg_promote()" can be used
to promote a PostgreSQL instance, however usage is restricted to
superusers and users to whom explicit execution permission for this
function has been granted.

Therefore, if execution permission is not available, fall back to
"pg_ctl promote".

doc/appendix-release-notes.xml

@@ -88,6 +88,16 @@
             </para>
           </listitem>
 
+          <listitem>
+            <para>
+              <link linkend="repmgr-standby-promote"><command>repmgr standby promote</command></link>
+              will check if the <literal>repmgr</literal> user has permission to execute
+              <function>pg_promote()</function> and fall back to <command>pg_ctl promote</command> if
+              necessary.
+            </para>
+          </listitem>
+
+
           <listitem>
             <para>
               Fix situation where replication connections were not created correctly, which