roles/nginx: Update comment for gzip_comp_level

From the HTML 5 Boilerplate server configs, see:

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

roles/nginx/files/nginx.conf

@@ -57,7 +57,12 @@ http {
     # Avoids the issue where a non-gzip capable client (which is extremely rare
     # today) would display gibberish if their proxy gave them the gzipped version.
     gzip_vary on;
+
+    # Compression level (1-9).
+    # 5 is a perfect compromise between size and CPU usage, offering about
+    # 75% reduction for most ASCII files (almost identical to level 9).
     gzip_comp_level 5;
+
     gzip_min_length 860;
     gzip_disable "msie6";
     gzip_http_version 1.1;