The term was coined by Carroll in the early 1980s. Anchor the tool in the task domain. Support error recognition and recovery. Support reading to do, study and locate. The central principle is task-orientation.
Brevity is a key element, but only because it can aid task-oriented activity.
Documentation is translated into many languages. There are differences between the way English is used in the USA, Britain and Australia for example. If your software is for an international market, then it makes sense to avoid ambiguities caused by differences in language usage. Then consider readers who are not native English speakers.