hier(7): adjust overlays/ subdirectory

Device tree overlays are installed in /boot/dtb/overlays by default.
Adjust the comment to mention fdt_overlays and loader.conf, but do not
repeat what is said in the parent directory's description.

PR: 261349
Reviewed by: grahamperrin, kevans
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40785