doc: Document that only superusers can use pg_import_system_collations().

Back-patch to v10 where pg_import_system_collations() was added.

Author: Atsushi Torikoshi
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/b7f484692a3e283710032e68b7f40617@oss.nttdata.com

doc/src/sgml/func.sgml

@@ -20211,6 +20211,7 @@ postgres=# SELECT * FROM pg_walfile_name_offset(pg_stop_backup());
     be <literal>pg_catalog</literal>, but that is not a requirement;
     the collations could be installed into some other schema as well.
     The function returns the number of new collation objects it created.
+    Use of this function is restricted to superusers.
    </para>
 
   </sect2>