doc: Mention cost-based delays for total_[auto]{vacuum,analyze}_time

doc: Mention cost-based delays for total_[auto]{vacuum,analyze}_time

30a6ed0ce4b has added four attributes to pg_stat_all_tables to track the
cumulative time spent in [auto]vacuum and [auto]analyze.  It was not
mentioned that the vacuum cost-based delays are included in these
numbers, which could be confusing now that the delays are included in
the vacuum progress view (bb8dff9995f2).

This commit adds an extra note about this matter.

Reported-by: Magnus Hagander <magnus@hagander.net>
Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/CABUevEz9v1ZNToPyD98JnWDGZgG=SmPZKkSNzU9hXQ-nGTQF0g@mail.gmail.com