hier(7): adjust overlays/ subdirectory

[mhorne]

Summary

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

[grahamperrin]

Confirming the move of /boot/overlays to /boot/dtb/overlays, from 2018:

• Move /boot/overlays to /boot/dtb/overlays · freebsd/freebsd-src@d5a390e · GitHub
• Amend missed reference to /boot/overlays, moved to /boot/dtb/overlays · freebsd/freebsd-src@2c58a19 · GitHub
• Add /boot/overlays to ObsoleteFiles · freebsd/freebsd-src@d96c11b · GitHub.

[kevans]

Whoops, thanks!