ProgrammableWeb – The Six Pillars of Complete Developer Documentation:
"A complete documentation set should try to include the following:
Class Reference: A comprehensive listing of API functionality.
Changelog: A reference of what changes in each API version.
Code Samples: A set of examples showing typical API usage.
Code Playground: An interactive explorer for trying the API live in the browser.
Developers Guide: A conversational written guide to using the API.
Articles: Tutorials and screencasts discussing different ways of using the API."
(Via Thorsten Mann.)