Whether it is a note for the office staff about the job, a reason for cancellation, or even simply a heads up about the work performed, comments are a regular part of the mobile app work flow.
From the Job view, tap the "Comments" tile.
On the next screen tap the "Add Comment" button along the bottom.
Type your message in the text box that appears. Once complete, tap "Save" in the top right corner. Your comment is now added to the job.