add a doc block to no-default

and remove the default_server direcdtive from example.com. Since this file
is likely to be the copy-and-paste file for multiple server configs,
don't mark example.com as the default.

sites-available/example.com

@@ -14,9 +14,9 @@ server {
 }
 
 server {
-  # listen 80 default_server deferred; # for Linux
-  # listen 80 default_server accept_filter=httpready; # for FreeBSD
-  listen 80 default_server;
+  # listen 80 deferred; # for Linux
+  # listen 80 accept_filter=httpready; # for FreeBSD
+  listen 80;
 
   # The host name to respond to
   server_name example.com;

sites-available/no-default

@@ -1,3 +1,10 @@
+# Drop requests for unknown hosts
+#
+# If no default server is defined, nginx will use the first found server.
+# To prevent host header attacks, or other potential problems when an unknown 
+# servername is used in a request, it's recommended to drop the request 
+# returning 444 "no response".
+
 server {
   listen 80 default_server;
   return 444;