doc: Mention that toast_tuple_target affects also column marked as Main.

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

doc/src/sgml/ref/create_table.sgml

@@ -1359,10 +1359,11 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
     <listitem>
      <para>
       The toast_tuple_target specifies the minimum tuple length required before
-      we try to move long column values into TOAST tables, and is also the
-      target length we try to reduce the length below once toasting begins.
-      This only affects columns marked as either External or Extended
-      and applies only to new tuples; there is no effect on existing rows.
+      we try to compress and/or move long column values into TOAST tables, and
+      is also the target length we try to reduce the length below once toasting
+      begins. This affects columns marked as External (for move),
+      Main (for compression), or Extended (for both) and applies only to new
+      tuples. There is no effect on existing rows.
       By default this parameter is set to allow at least 4 tuples per block,
       which with the default block size will be 2040 bytes. Valid values are
       between 128 bytes and the (block size - header), by default 8160 bytes.