Update nginx_example.conf

Elaborate on reason for 7M as client_max_body_size, although it is configured as 3*1024*1024.

qiita_pet/nginx_example.conf

@@ -3,7 +3,7 @@ events {
 }
 
 http {
-    client_max_body_size 4M;  # increase maximum body size from default 1M to match https://github.com/qiita-spots/qiita/blob/ac62aba5333f537c32e213855edc39c273aa9871/qiita_pet/static/vendor/js/resumable-uploader.js#L51
+    client_max_body_size 7M;  # increase maximum body size from default 1M to match https://github.com/qiita-spots/qiita/blob/ac62aba5333f537c32e213855edc39c273aa9871/qiita_pet/static/vendor/js/resumable-uploader.js#L51 (which is 3M). Note that resumable-uploader.js's last chunk can be max. twice as large as chunk size, see: https://github.com/23/resumable.js/issues/51 
 
     # ports to redirect for mainqiita
     upstream mainqiita {