doc: Update pg_upgrade documentation to match recent description changes.

Commit 792353f7d5 updated the pg_dump and pg_dumpall documentation to
clarify which statistics are not included in their output. The pg_upgrade
documentation contained a nearly identical description, but it was not updated
at the same time.

This commit updates the pg_upgrade documentation to match those changes.

Backpatch to v18, where commit 792353f7d5 was backpatched to.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Bruce Momjian <bruce@momjian.us>
Discussion: https://postgr.es/m/CAHGQGwFnfgdGz8aGWVzgFCFwoWQU7KnFFjmxinf4RkQAkzmR+w@mail.gmail.com
Backpatch-through: 18

doc/src/sgml/ref/pgupgrade.sgml

@@ -833,10 +833,10 @@ psql --username=postgres --file=script.sql postgres
     <para>
      Unless the <option>--no-statistics</option> option is specified,
      <command>pg_upgrade</command> will transfer most optimizer statistics
-     from the old cluster to the new cluster.  However, some statistics may
-     not be transferred, such as those created explicitly with <xref
-     linkend="sql-createstatistics"/> or custom statistics added by an
-     extension.
+     from the old cluster to the new cluster.  This does not transfer
+     all statistics, such as those created explicitly with
+     <xref linkend="sql-createstatistics"/>, custom statistics added by
+     an extension, or statistics collected by the cumulative statistics system.
     </para>
 
     <para>