it already is in manual:
https://docs.ankiweb.net/#/files?id=startup-options
If you have made a destructive change on one computer and have an undamaged copy on another computer, you may wish to start Anki without syncing in order to use the full sync option without first downloading the changes. Similarly, if you are experiencing problems with Anki, you might want to (or might be instructed to) disable add-ons temporarily to see if one might be causing the problem. You can do both of these things by holding down the Shift key while starting Anki.