linuxkpi: Document why <linux/cpufeaure.h> has an include commentted out

<linux/cpufeature.h> on Linux includes <linux/init.h>. We don't have the
latter on FreeBSD yet, so let's comment this include for now. It is
still referenced because sometimes, consumers of headers rely
voluntarily or not on the namespace pollution.

Reported by: jrtc27
Approved by: jrtc27
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49448