Documentation in SonarQube 4.4+
Starting with SonarQube 4.4, the web application offers a built-in and always-up-to-date documentation of the Web Service API. The link to access it is located in the footer of the page:
You can see it live on Nemo at http://nemo.sonarqube.org/api_documentation.
Documentation for older versions
For SonarQube < 4.4, see the archived documentation.