doc: Add secondary index entries for vacuum-related parameters.

doc: Add secondary index entries for vacuum-related parameters.

For parameters that exist as both configuration and storage options,
the documentation typically includes secondary index entries to
help users distinguish and locate the relevant references easily.

However, such index entries were missing for vacuum_truncate and
vacuum_max_eager_freeze_failure_rate, both introduced in v18.

This commit adds appropriate secondary index terms for these parameters
to ensure consistency with other parameters and improve usability of
the documentation index.

Author: Fujii Masao <[email protected]>
Discussion: https://postgr.es/m/e95c899a-2aeb-45b7-8fd3-7a27dcdb475b@oss.nttdata.com