re-establish section on why __init__ not called on load

re-establish section on why __init__ not called on load

this section got lost, leaving the FAQ to point to an
empty document.  Rewrite a new section introducing that
__init__ is not called on load, illustrate strategies.

I am not that happy with *where* this doc is, as this is
supposed to be "mapping styles" high level introductory
type stuff, but there's nowhere else for it.

References: https://github.com/sqlalchemy/sqlalchemy/discussions/10923
Change-Id: Ie9260e4076bc82da0ef6dc11349a85beb0223a33