Short version: try adding pins with custom notes to multiple stars on the star map, and clicking around to view different notes about different pins. The notes will be smashed and overwritten. Help!
Click tab to open the overlay and click on the Sector Map sub-tab.
Zoom in to be near your current location.
Click on a star and click the pin icon (target symbol with plus inside a circle). Select a color for the pin and add custom notes and save.
Click on a different star and click the pin icon, and again, select a color + add custom notes.
Do this a few times, using different custom notes for each star/pin.
Each star/pin has its own custom notes and they don't overwrite one another. Clicking around on the map at old and new pins doesn't change the custom note text. Custom note text is only changed when you explicitly click to modify the text associated with that star/pin.
Custom notes for subsequent stars/pins are overwritten when you add one note to one star/pin then click around to other stars/pins. You cannot revert to get your original notes back, they are lost for good.