If you didn't know about Confluences inline comments already you've been missing out.
The Diary also supports this feature to let you easily comment on entries.
To use the inline commenting feature just highlight the part of the entry that you want to comment and click on the "Add inline comment"-button that should appear above your selection.
On the right hand side of your screen there should appear a small editor allowing you to write your comment.
After you are done writing your comment simply save it to have your comment displayed on the page.
So if you read the whole "Using Diary for Confluence" guide and ended up here, you should now know everything about the Diary.
If you still want to read more about Diary for Confluence perhaps take a look at the FAQ.