docs: Rework sections on partition pruning/exclusion

docs: Rework sections on partition pruning/exclusion

Explain partition pruning more thoroughly, in a section above the one
that explains constraint exclusion, since the new feature is the one
that will be used more extensively from now on.  Move some of the
material from the constraint exclusion subsection to the one on
partition pruning, so that we can explain the legacy method by
explaining the differences with the new one instead of repeating it.

Author: David Rowley, Álvaro Herrera
Reviewed-by: Amit Langote, David G. Johnston, Justin Pryzby
Discussion: https://postgr.es/m/CAKJS1f8PECxEi1YQ9nhVtshtfOMHUzAMm_Zp4gGCOCnMPjEKJA@mail.gmail.com