Provide introduction for the arch(7) manpage.

Start with some words about linear address space and its layout, then
explain pointers models and ABIs, providing explanation to the
structure of the tables.

Reviewed by: emaste, imp
'Future-proof' cheri wording by: brooks
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D10596