hier.7: polish entry to system manual

Entries reworded to improve grammar or add keywords:

• document description: substitute layout for index for first-glance
• /etc/freebsd/update.conf
• /usr/share/vi/

Entries that have been wordshuffled for brevity or consistency:

• /{bin,libexec,nonexistent,sbin,tmp}/
• /boot/dtb/overlays/
• /boot/{defaults,efi,kernel,lua}/
• /usr/libexec/
• /var/{db,games,lib,log/{bsdisks.log,spool/,tmp/}}
• /usr/share/sysroot/VERSION/MACHINE.MACHINE_ARCH/ (unfold a little)
• /usr/local/share/doc/freebsd/ (unfold a little)

Entries that have macro or linking adjustments:

• /{boot,boot/efi,dev,etc,home,sbin,usr/bin}/ (overview or intro pages)
• /media/ (bsdisks is from ports, but afaik pulled by every desktop)
• /usr/share/{lib/,lib/dtrace/,libdata/games/,man/}/ (intro pages)
• /var/log/messages (syslog(3) -> syslogd(8))

Entries that have been added:

• /dev/{cuaU0,vmm,zvol}/
• /usr/share/{atf,bhyve}/
• /usr/share/libexec/hyperv/ (thanks @rtprio)
• /local/<subdirectories>/
• /var/log/debug.log (thanks @pauamma)
• /var/spool/{lock,lpd}

Please forgive my earlier mistakes fixed in this commit:

• /{dev,/usr/share/{calendar,misc}} : accidentally reverted description
• /etc/local-unbound.conf/ (wrong link)
• /usr/{freebsd-dist,lib32,libdata},/var/log/{dmesg.today,lpd-errs} (typos)
• /{usr/libdata/gcc/,var/log/bsdisks.log} (removed, unnecessary)

PR: 261349
Reviewed by: mhorne
MFC after: 1 week
Pull Request: https://github.com/freebsd/freebsd-src/pull/1131