roles/nginx: Add comment for sendfile option

From: https://github.com/h5bp/server-configs-nginx/blob/master/nginx.conf

Signed-off-by: Alan Orth <alan.orth@gmail.com>

roles/nginx/files/nginx.conf

@@ -23,6 +23,11 @@ http {
 
     access_log off;
 
+    # Speed up file transfers by using sendfile() to copy directly
+    # between descriptors rather than using read()/write().
+    # For performance reasons, on FreeBSD systems w/ ZFS
+    # this option should be disabled as ZFS's ARC caches
+    # frequently used files in RAM by default.
     sendfile        on;
     #tcp_nopush     on;