Add remark about why we use non-default DNS

roles/common/files/resolved-arch.conf

 # Query authoritative namesevers directly when resolving
 # Arch Linux domains so we instantly know about Arch-specific changes.
+# The default nameservers provided by Hetzner DHCP are very slow to pick up such changes.
 [Resolve]
 DNS=213.239.242.238       # ns1.first-ns.de
 DNS=2a01:4f8:0:a101::a:1  # ns1.first-ns.de