Three types of comments exist: Reader, reviewer, and change comments.
Reader comments
You can add a comment by clicking on Add Comment, typing the comment's content in the field, and clicking on the validation icon.
It is also possible to add a comment by highlighting the text that you want to comment on and clicking on Add a comment. This will copy the text selected into the comment section.
Reader comments are visible to other readers as well as in the editor by clicking on Monitor --> Reader comments:
Note: If you cannot add a comment, it is because your admin deactivated comments for all or certain manuals.
If you are interested in learning more about other types of comments, feel free to check out Review comments and Change log comments.