Timers and diaper change buttons have a notes button next to them to edit notes that should accompany a logged time.
Pressing the button will unfolf a text-editor that allows one to take notes for the corresponding diaper change or timer:
Notes will be persisted locally on the Android device until the timer is logged. Keep that in mind to not loose data. After a timer is stopped, the notes are cleared. If you change your mind, and do not want to store taken notes with your timer, press the notes-button again. Notes will be persisted locally but when stopping the timer they will not stored with the logged event.