libpq doc: Clarify what commands return PGRES_TUPLES_OK

The old text claimed that INSERT and UPDATE always return
PGRES_COMMAND_OK, but INSERT/UPDATE with RETURNING return
PGRES_TUPLES_OK.

Josh Kupershmidt

doc/src/sgml/libpq.sgml

@@ -2469,7 +2469,8 @@ ExecStatusType PQresultStatus(const PGresult *res);
         command that happens to retrieve zero rows still shows
         <literal>PGRES_TUPLES_OK</literal>.
         <literal>PGRES_COMMAND_OK</literal> is for commands that can never
-        return rows (<command>INSERT</command>, <command>UPDATE</command>,
+        return rows (<command>INSERT</command> or <command>UPDATE</command>
+        without a <literal>RETURNING</literal> clause,
         etc.). A response of <literal>PGRES_EMPTY_QUERY</literal> might
         indicate a bug in the client software.
        </para>