How to add or insert note in Google Docs spreadsheet?
Notes can be added to the cell using Notes option.
Option 1: Right click on the cell and select "Insert note".
Option 2: Click Insert → Note or press Shift F2
Step 2: Enter the Notes to be added. Cell will be marked with the small black arrow at the top right corner.
Step 3: Whenever you mouse over the cell, it will display the notes as shown below,