Previously it was documented that toast_tuple_target affected column
marked as only External or Extended. But this description is not correct
and toast_tuple_target affects also column marked as Main.

Back-patch to v11 where toast_tuple_target reloption was introduced.

Author: Shinya Okano
Reviewed-by: Tatsuhito Kasahara, Fujii Masao
Discussion: https://postgr.es/m/93f46e311a67422e89e770d236059817@oss.nttdata.com