For some time, there has been a solution around for Project Server users, allowing them to enter their comments for a specific task per day. This takes place from the default timesheet of Project Server and now finally also in Project Online.
For those, who are using timesheets as the foundation for invoicing, this is a must have app. It dramatically reduces your need for planning complexity, when team members are allowed to add some text to any timesheet entry. Better yet, all comments are stored in the standard timesheet table of Project Online/Server, meaning you can report on the data using the out-of-the-box ODATA feeds.
Get the app here: Link