minor replication slot docs edits

Pursuant to a comment I made a few months ago[1], I propose the attached
changes to replication slots documentation.  In essence, I want to
explain that replication slots are good, and the max_size GUC, before
moving on to explain that the other methods are worse.

Thanks

[1] https://postgr.es/m/20230413111838.e7yxke2dtwrxw3qy@alvherre.pgsql

-- 
Álvaro Herrera         PostgreSQL Developer  —  https://www.EnterpriseDB.com/
"Entristecido, Wutra                     (canción de Las Barreras)
echa a Freyr a rodar
y a nosotros al mar"