Adding a Comment to a Message in a Doc:

To add a comment to a message in a doc, you must first navigate to the doc you want to comment on.

When you mouse over each message field in the doc, you will see the comment icon appear.

Just click on the Comment icon.

 GrokSpark Screenshot

Then add your comments into the popup window and click the Send Feedback button when you are done.

 GrokSpark Screenshot

Once a comment has been sent, the message owner will receive a notification via the notification bell icon.

For more on viewing comments on messages.