nginx: add comment explaining repo rate limiting

conf/server-base.conf

@@ -26,6 +26,9 @@ location /realtime {
     proxy_request_buffering off;
 }
 
+# At the begining and end of a push/pull, /v1/repositories is hit by the Docker
+# client. By rate-limiting just this endpoint, we can avoid accidentally
+# blocking pulls/pushes for images with many layers.
 location /v1/repositories/ {
     proxy_buffering off;