Usability is the combination of effectiveness, efficiency, and satisfaction with which the users accomplish defined goals in a given environment. User-centered documentation matches the users' mental model, thereby helping the users find information they want quickly and easily in their hour of need.
The list of documentation usability criteria is fairly subjective at this time, and various opinionated discussion groups have contributed to this. Usable documentation is based on a deep understanding of the users' tasks, and this understanding can only be gained through interviewing representative users. Applying information architecture techniques, the content within documentation should be properly chunked so that the users can assimilate the information properly. Procedural guides should have a well-defined and searchable index that enables users to connect key application terms to their correct context.
User-friendly documentation is always succinct, but never at the expense of omitting critical/useful information. It should be developed using a structured process so that it starts with the big picture and gradually adds lower level of details, addressing the needs of every unique group of users. Finally, the documentation must be tested among a representative group of users, and their feedback should be incorporated to make sure that it has met all of the major usability criteria.