kernel: Clarify kern.elfNN.nxstack sysctl description

The nxstack sysctl controls processing of the PT_GNU_STACK segment,
not directly whether or not the stack is executable.

Reviewed by: kib, shurd
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48221

(cherry picked from commit 837feb4d05c2dccafa1698649b58f7b7fdc59c54)