doc: Correct setting restrictions for vacuum_max_eager_freeze_failure_rate

doc: Correct setting restrictions for vacuum_max_eager_freeze_failure_rate

The documentation incorrectly stated that this parameter could only
be set in postgresql.conf or on the server command line. However, it
is a PGC_USERSET parameter and can therefore be changed by any user
within a session.

Remove the incorrect restriction while retaining the note about the
per-table storage parameter override.

Backpatch to v18, where this parameter was introduced.

Author: Afrah Razzak <mypg.afrah@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CAAJ6gzGeup6WPpraF1g6iWYOQsvLmhKk94acJ=-iq+Gg_x9W0w@mail.gmail.com
Backpatch-through: 18