Improve documentation for connection pool logging

Improve documentation for connection pool logging

Also do a general pass for logging + doc formatting,
add more cross-linking and remove obsolete information such
as "echo_uow"/

Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Fixes: #4571
Closes: #4583
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4583
Pull-request-sha: e98ad34eca7e9f59fb07cd8b7ec317c1cb989848

Change-Id: I03f7354a4ef55fd8b6a51d03a280579f36e8a06c
(cherry picked from commit 2b25827b8455aaaa8c73180d0c26afb3e3187adc)