Note that sys/systm.h is special too

If you need / want to includerd sys/systm.h, it has to be just after
param.h/types.h. Document this existing practice. Not all kernel files
include systm.h, but when you do, it should be done out of order.

Reviewed by: vangyzen, kib, emaste
Differential Review: https://reviews.freebsd.org/D26981