Software documentation is written text that accompanies computer software. It either explains how it operates or how to use it, and may mean different things to people in different roles. wikipedia.org

Software testing is an investigation conducted to provide stakeholders with information about the quality of the product or service under test. wikipedia.org

Usability testing is a technique used in user-centered interaction design to evaluate a product by testing it on users. This can be seen as an irreplaceable usability practice, since it gives direct input on how real users use the system. wikipedia.org