Currently the only ARCH documentation is placed before the IGNORE
variables are introduced.

This patch swaps the order of those two sections and pushes "Marking
a Port as Architecture Neutral" down one level.

By itself this patch seems trivial; however, it needs to be done to
prepare for expanding the section with non-trivial material.

Approved by: mat
Differential Revision: https://reviews.freebsd.org/D19233