Doc: enhance the description on connection_life_time

Doc: enhance the description on connection_life_time

connection_life_time is a config value to determine the life time of
cached connections to PostgreSQL backend. Current document lacks a
description that the expiration calculation is actually done at the
time when the client disconnects to the process which holds the cached
connections.

Discussion: [pgpool-hackers: 4577] Doc: enhance the description on connection_life_time
https://www.pgpool.net/pipermail/pgpool-hackers/2025-April/004578.html
Backpatch-through: v4.2