Minor documentation fixes related to v18

Hi,

I'd like to propose three minor documentation fixes related to parameters
introduced in v18. Since they align with new content in that version,
I'm thinking to apply them to v18. Thoughts?

0001:
For parameters that exist as both configuration and storage options,
the documentation usually includes secondary index entries to help users
find them easily. However, such entries were missing for vacuum_truncate
and vacuum_max_eager_freeze_failure_rate. This patch adds the appropriate
index terms.

0002:
The description of track_cost_delay_timing lists where cost-based vacuum
delay timing information is shown when the setting is enabled. Although
this information also appears in ANALYZE VERBOSE output, that wasn't
mentioned. This patch adds it to the list.

0003:
The io_workers parameter was documented with the type "int", but
the standard format in our docs is "integer". This patch updates it
for consistency with other GUC parameter descriptions.

Regards,

-- 
Fujii Masao
NTT DATA Japan Corporation