Fix documentation of pg_start_backup and pg_stop_backup functions.

This commit adds the description that "non-exclusive" pg_start_backup
and pg_stop_backup can be executed even during recovery. Previously
it was wrongly documented that those functions are not allowed to be
executed during recovery.

Back-patch to 9.6 where non-exclusive backup API was added.

Discussion: https://postgr.es/m/CAHGQGwEuAYrEX7Yhmf2MCrTK81HDkkg-JqsOUh8zw6+zYC5zzw@mail.gmail.com

doc/src/sgml/func.sgml

@@ -19835,6 +19835,8 @@ SELECT set_config('log_statement_stats', 'off', false);
     The functions shown in <xref
     linkend="functions-admin-backup-table"/> assist in making on-line backups.
     These functions cannot be executed during recovery (except
+    non-exclusive <function>pg_start_backup</function>,
+    non-exclusive <function>pg_stop_backup</function>,
     <function>pg_is_in_backup</function>, <function>pg_backup_start_time</function>
     and <function>pg_wal_lsn_diff</function>).
    </para>