clarify alternative mapping example

clarify alternative mapping example

this second example is not part of the doctest steps,
clarify that it's not part of code examples to be present
in execution steps.   Add an extra registry +
declarative base on top
so that even if someone does run it, the Base will have
been reset and the examples will continue to work
(noting that column order in statements may change, but
probably nothing else).

Fixes: #7891
Change-Id: Icb1ba310230841e502185d9d0cadd3c18d467292
(cherry picked from commit 1dffb7cedeb009ca6c532db558bd0588dd846957)