There are actually two steps in accomplishing this.
First you need to enable the comment section in the datasource. Then you assign the required comment option to each user, to add or NOT add comments.
To enable the comment section in the datasource do the following:
Next assign the option to each individual user to add comments by doing the following:
Note that when you log in as the user with the required comment option enabled, you see your comments in the audit tab. Now if you log in as a different user (without required comment enabled) you only see that the file was checked in, but no comment was entered.