@Maker good call, and good reference and wink on ISO9241. I think good documentation is vital, whether inline or separate like a manual. I also think most contexts should allow base usability without needing to use documentation.

A round door knob is intuitive once one is shown but only if one is not differently limbed. However, a door handle (like pictured) even some animals may be able to figure out with no instruction or example. It is more accessible and typically more usable.