OptionalObsoleteFiles.inc: document block order

Options should be in sort(1) order by primary option (usually, but not
always, first in the if statement).

Reviewed by: emaste, imp, netchild
Pull Request: https://github.com/freebsd/freebsd-src/pull/896

(cherry picked from commit 4cd0f014a1285c1b57e583b260282ca433a8ccdd)