Add documentation about /etc/hosts preloading

doh-client/doh-client.conf

@@ -25,6 +25,8 @@ upstream_ietf = [
 # Bootstrap DNS server to resolve the address of the upstream resolver
 # If multiple servers are specified, a random one will be chosen each time.
 # If empty, use the system DNS settings.
+# If you want to preload IP addresses in /etc/hosts instead of using a
+# bootstrap server, please make this list empty.
 bootstrap = [
 
     # Google's resolver, bad ECS, good DNSSEC