You can now configure custom send back workflow notifications and include the entered send back comments.  Within an Approval or Task workflow step you'll now see a 'Customize send back email notification' option:

Once enabled you can customize the message (subject and body) of the notification that will send when the send back action is taken.  You can also include the new 'Sendback Comment' variable to include the comments entered by the reviewer on the send back action:

The email sent will include the customized subject/body text including the send back comments:

More information on how to configure can be found in the Approval Step and Task Step workflow articles.