docs: Clarify that NULL arg to set_config() means reset to default

Author: David G. Johnston <david.g.johnston@gmail.com>
Reviewed-by: Zhang Mingli <zmlpostgres@gmail.com>
Discussion: https://www.postgresql.org/message-id/CAKFQuwY0SK6JdCci1VJX6xsztRXgGeVEY-grkENZx%2B3CZpyPcQ@mail.gmail.com

doc/src/sgml/func.sgml

@@ -28576,6 +28576,11 @@ acl      | {postgres=arwdDxtm/postgres,foo=r/postgres}
         use <literal>false</literal> instead. This function corresponds to
         the SQL command <xref linkend="sql-set"/>.
        </para>
+       <para>
+        <function>set_config</function> accepts the NULL value for
+        <parameter>new_value</parameter>, but as settings cannot be null, it
+        is interpreted as a request to reset the setting to its default value.
+       </para>
        <para>
         <literal>set_config('log_statement_stats', 'off', false)</literal>
         <returnvalue>off</returnvalue>