doc: Update pg_upgrade documentation to match recent description changes.

doc: Update pg_upgrade documentation to match recent description changes.

Commit 792353f7d52 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 792353f7d52 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