doc: Fix description of pg_replication_origin_oid() in error case

This function returns NULL if the replication origin given in input
argument does not exist, contrary to what the docs described
previously.

Author: Ian Barwick
Discussion: https://postgr.es/m/CAB8KJ=htJjBL=103URqjOxV2mqb4rjphDpMeKdyKq_QXt6h05w@mail.gmail.com
Backpatch-through: 10

Author: michaelpq

doc/src/sgml/func.sgml

@@ -26524,7 +26524,8 @@ postgres=# SELECT * FROM pg_walfile_name_offset(pg_stop_backup());
        </para>
        <para>
         Looks up a replication origin by name and returns the internal ID. If
-        no such replication origin is found an error is thrown.
+        no such replication origin is found, <literal>NULL</literal> is
+        returned.
        </para></entry>
       </row>