add comments about aws-sdk and s3 backends

services/filestore/app/coffee/AWSSDKPersistorManager.coffee

@@ -1,3 +1,8 @@
+# This  module is not used in production, which currently uses
+# S3PersistorManager.  The intention is to migrate S3PersistorManager to use the
+# latest aws-sdk and delete this module so that PersistorManager would load the
+# same backend for both the 's3' and 'aws-sdk' options.
+
 logger = require "logger-sharelatex"
 aws = require "aws-sdk"
 _ = require "underscore"

services/filestore/app/coffee/S3PersistorManager.coffee

@@ -1,3 +1,7 @@
+# This module is the one which is used in production.  It needs to be migrated
+# to use aws-sdk throughout, see the comments in AWSSDKPersistorManager for
+# details. The knox library is unmaintained and has bugs.
+
 http = require('http')
 http.globalAgent.maxSockets = 300
 https = require('https')